I have taken the cover off, and made sure the thermostat dial was set to maximum (it was). My legionella cycle times out and causes an error (on my Samsung controller) as it can never get up to the desired temperature, as the immersion cuts out below 60C.

An immersion heater should have two thermostatic switches in series.

One is fixed and is an emergency fail-safe.
It should be set by the manufacturer at a level well above the specified range for that immersion heater,
and well below 100°C.

The second thermostat is variable.

If you want water temperature to reach 60°C, then the upper-limit of the variable thermostat should be above this.
Otherwise it will only measure the water in the immediate proximity of the heating elements.

It's not always possible to see the safety thermostat, or put a multi-meter onto its terminals.
But where this is the case, it would be useful to know which thermostat has opened its contacts.

In the above photo, the safety thermostat is at the top with two white wires attached.

In the majority of cases, the Heat Pump itself doesn't sense what the immersion heater is doing.
Instead it uses an entirely separate thermal sensor, elsewhere on the side of the tank.

If the Samsung system is producing an error after a period of time, then that suggests the immersion heater itself stopped supplying heat much earlier.

Once the white plastic cap is removed, my immersion looks like the image posted on page 3 of this thread, with a red slider and a + and - scale. The slider is set at the maximum + end of the scale.

You are correct that the system has a temperature sensor fitted in a pocket in the tank which reads the DHW temperature. The only way I know the immersion has cut out is by watching the power draw on my smart meter IHD (or now on my Tesla Powerwall app) to monitor the house load - it's easy to spot an additional 3kW load switching either on or off.

The Samsung has a time limit setting for the legionella cycle. Because the immersion cuts out below 60C, it will just sit there waiting for it to reach 60C (which will never happen), and then time out with an error once the time limit is reached. The immersion cuts out pretty quick as it only gets to operate from when the heat pump cuts out at 54-55C up until it cuts out itself at 57-58C. Obviously if the temp falls due to heat loss or hot water usage during the legionella cycle time period, the immersion would kick back in again and then cut back out again once it hits temperature (57-58C).

I did think about contacting the company to come back and replace the immersion, but decided against it as I'm never going to use it other than in an emergency to heat hot water, and in an emergency I'd only ever heat to ~45C on immersion, and it's working fine for that usage scenario.

EDIT - of course the 'error' could be with the temp sensor in the tank reading too low; reading 57-58C when the tank hot water temp is actually 5C higher and the immersion is operating to spec all along, or even a combination of both the temp sensor reading a degree or two low and the immersion cutting out a degree or two low aswell.

Sorry, I should have updated the thread but there was so much backwards and forwards between Mitsubishi and my installer that I forgot.

My legionella timing problem was resolved by adjusting the thermostat on a replacement Immersion heater and ensuring that the DHW at the start is at the usual 50c. The legionella cycle finished within a 2 hour window.

I have taken the cover off, and made sure the thermostat dial was set to maximum (it was). My legionella cycle times out and causes an error (on my Samsung controller) as it can never get up to the desired temperature, as the immersion cuts out below 60C.

An immersion heater should have two thermostatic switches in series.

One is fixed and is an emergency fail-safe.
It should be set by the manufacturer at a level well above the specified range for that immersion heater,
and well below 100°C.

The second thermostat is variable.

If you want water temperature to reach 60°C, then the upper-limit of the variable thermostat should be above this.
Otherwise it will only measure the water in the immediate proximity of the heating elements.

It's not always possible to see the safety thermostat, or put a multi-meter onto its terminals.
But where this is the case, it would be useful to know which thermostat has opened its contacts.

In the above photo, the safety thermostat is at the top with two white wires attached.

In the majority of cases, the Heat Pump itself doesn't sense what the immersion heater is doing.
Instead it uses an entirely separate thermal sensor, elsewhere on the side of the tank.

If the Samsung system is producing an error after a period of time, then that suggests the immersion heater itself stopped supplying heat much earlier.

I doubt that installers would want to hang around to test for this, and hence it won't feature on the Commissioning Check Sheet.

If consumers were aware of this situation, then they should look on the Commissioning Chart to see what temperatures and timings were entered by the installer for the anti-Legionella Cycle.
No data recorded on the chart would be a reason to reject the installation until the system is properly operational.

This is a Building Regulations requirement from Part-L of the Approved Documents.

The consumer is within their rights to insist that the anti-legionella cycle is properly installed, tested and effective, irrespective of what the Installer, HP Manufacturer and MCS regulations state.

I have a Joule pre-plumbed tank with a very similar looking 3kW immersion heater fitted. I have taken the cover off, and made sure the thermostat dial was set to maximum (it was). My legionella cycle times out and causes an error (on my Samsung controller) as it can never get up to the desired temperature, as the immersion cuts out below 60C. I suspect this is just down to a poorly calibrated thermostat on the immersion heater. I can't remember exactly where it cuts out as we've since disabled the legionella cycle, but somewhere around 57-58C if I recall correctly, so this is the hottest we are able to heat the DHW tank (the ASHP will get it to a max of around 54-55C).

My legionella cycle would run for the prescribed length of time, never reach the allotted temperature and cause an time out error locking the system resulting in no space heating until the error had been manually cleared.

Maybe the same is happening in @richard24738 case (I don't know if this was ever resolved)?

The crucial thing to note is that it shouldn't go through several right-angle bends.
Either it needs to be bends formed in the pipe itself, or else uses swept-elbow fittings.

They were more than just "a bit miserly" with the pipe insulation.
They've breached the regulations.

The rule is that all pipes should be insulated within a metre of a hot water tank.
It doesn't matter whether the pipe is meant to carrying cold-input, or if it's empty, such as the pressure release system.
The pipe still needs insulating because the copper 'sucks' energy out of the tank.

Below the Tundish the pipe goes to the exterior, the 2nd pic shows what is above.

They were a bit miserly with insulation, I have added quite a bit myself.

I'm going to add some over the hole where the temperature probe is as I can feel warmth when I put my finger in, there is another as well near the top of the tank. Thank you for the heads up.

I emailed Mitsubishi on Sunday about the Legionella cycle, haven't heard back yet though, will try calling them when I get chance. Another thought is to set the cycle temp to 59 instead of 61 and hopefully it will end the program when it reaches 59.5.