worcester 350 combi boiler

[h1spm]

I have been having a problem with my boiler in that recently occasionally I have not been getting hot water. This is an intermittant fault (see previous post).
Gucciman has kindly given me some pointers, since which I have carried out further checks. The observations are as follows:-

Demand light does not come on when taps turned on, even when water is hot (coincidence?)

It appears that the taps need to be open more than they used to be to get hot water.

Has anybody (Including the ever helpful Gucciman) got any idea what the fault may be?

Thanks

[h1spm]

PS, today I noticed when running a sink of water, the water went cold after about half a sink full of hot.