Ed,

Finally I have been able to check diagnotsic mode 24 and it does flick
between 0 and 1 when hot water is being drawn.

Should I therefore assume that it is the APS that requires replacing?
Would this switch also be the cause of not being able to draw any hot
water for ages as well as it fluctuating in temperature?

Also, why does this not affect the central heating system which seems
to operate perfectly?

Thank you very much for your help. Like many others have found, you are
a life saver!

Ben