Ive had a look for the specs of your cylinder online, but unfortunately I cannot find any schematic detailing where a sensor might be sited. So whether this is a contributory factor or not I cant say.

Had a look yesterday and the DHW was set to 43C but the temp in the tank after the 2 hour schedule finished was 44.5C.  I've now set the schedule to 1 hour but am going to be out this aft when it finishes so will have to check the temp tomorrow.

That is more normal. The DHW heating cycle will turn off the moment the tank reaches the set point (43C in your case), but the circulation pumps continue to run for another minute or so, and the circulating water is likely 7-10C hotter (so maybe 50-53C) and some of that heat will transfer into the DHW tank after the set point has been reached, so it is normal and to be expected for the final temperature to end up 1-2C higher than the set point, but not 4-5C higher as per your original post.

What temp have you set for your heating water? could there be an issue with the 3 way valve / zone valve?

FWIW I think you are doing the right thing putting the DHW onto timed as it's better for COP to heat the tank from a cooler temp rather than continuously on a 5 deg hysteresis.

The booster is set to Use and the delay time is 75 mins.  Overshoot is 0C.

I've decreased the time the tank heats on the schedule down to 60 mins and will see how that goes.  This time is below the delay time of 75 mins for the booster anyway, so the booster even while on shouldn't affect anything.  As the overshoot is set to 0C that can't be responsible for heating the water higher than the set point.

I agree, it doesn't look like that was the culprit, but was worth checking.

Let us know how you get on.