It connects to here, definitely. I felt by reaching and traced back.

If I turn the blue to the right, the whoosh increases.

One thing that is winding me up generally also is this

it only explains what 6 of these 15 symbols are. I have the wee face with the shushing figure and from what I can see on the Grant YouTube channel this indicates Low Tarriff Night Mode. But I certainly didn’t turn that on and absolutely nowhere can I seem to find out what that is. 
It just appeared one day.

@marvinator80

I will have a look at the manual tomorrow.

Posted by: @marvinator80

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It connects to here, definitely. I felt by reaching and traced back.

-- Attachment is not available --

If I turn the blue to the right, the whoosh increases.

Is the water actually still flowing out?

That looks like your pressure relief valve doesn't get the chance to close back, have you tried to stop/close the water supply to the tank to allow the pressure to drop in the tank in order for the valve to close? If that's the case.

Might be one in a million change that it caught some debris/smudge that blocks it.

@fazel yeah, still flowing out. It made a noise a minute ago there like a locomotive horn! Turned the tap on and it stopped. 

there must be air in it somehow.

Posted by: @marvinator80

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still flowing out. 

Turned the tap on and it stopped. 

so it's stopped now after turning which tap on? or it stopped temporarily?

I just the pressure and then ran every tap on the house on hot for 5 mins and then turned them all off. It seems to have settled now. 

the noise has stopped. The outflow pipe is down to just a trickle and the temperate is now going up normally with hot water coming out of taps again. 

weird!!!!

@marvinator80 if the explanation from the video it's true in your case, the cause is fluctuation in water pressure coming from the supply, if this pushes out the valve, the valve doesn't close back until pressure drops under a certain level(close supply and open taps). If it will happens often you will have to call the plumber.

You may need a mains pressure regulator on the cold water inlet to your house. The pre-plumb cylinder inlet manifold has a 3bar pressure relief valve on the cold side. If your mains pressure increases much above that the valve could lift and pass pressure. https://www.grantuk.com/products/cylinders/quick-recovery/
A mains pressure regulator is normally fitted after your stop cock. It should prevent any unwanted operation of the PRV supplied with the Grant cylinder.

See attached page 29

for full description of Chofu controller features. Grant only give a redacted description in their user manual.

Posted by: @marvinator80

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One thing that is winding me up generally also is this

it only explains what 6 of these 15 symbols are. I have the wee face with the shushing figure and from what I can see on the Grant YouTube channel this indicates Low Tarriff Night Mode. But I certainly didn’t turn that on and absolutely nowhere can I seem to find out what that is. 
It just appeared one day.

It would appear that Night Time mode has been selected.

Check the settings shown in pages 42 to 46 in the attached manual.

Still trickling a bit today, intermittently.

It stops completely for a while whenever we use the hot water. Weird.

messaged the installers today but not heard back yet. Will call them tomorrow.

Posted by: @marvinator80

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Still trickling a bit today, intermittently.

It stops completely for a while whenever we use the hot water. Weird.

messaged the installers today but not heard back yet. Will call them tomorrow.

I think that as Fazel and AllyFish have pointed out, it would appear that the pressure relief valve is operating. If the mains water supply pressure is greater than the system normal operating pressure, then there should be a pressure reduction valve installed on the water supply to your system.

Do you have any pressure gauges on your system and is the pressure varying?

Please provide make and model details of any valves installed on your system.