RAVENHEAT ls100t - warm water only, heating ok?

Hello, I'm hoping someone may be able to help.
My combi boiler was fitted around 6 years ago, and after two replacement timers in the first year, it has since been totally reliable.
Well, reliable except for the continual need to top up to raise the pressure around once a week - no leak has ever been found in the ch system, so this has become routine, and not of huge concern.
However, a fault has developed - I am only getting luke warm water. For around a week now, with heating on or off, my water temperature never lifts beyond this level.
Having read many threads on this website, I have given a hefty thump to the top right corner of the boiler whilst running the hot tap, and this has worked on two of the three occasions, providing a chenge to the sound of the boiler, as though firing a further cylinder, and providing HOT water.
I cannot keep doing this however, as I may get a sore hand!!

Many threads seem to suggest changing the DHW thermistor - is this likely to be the solution?

I've seen these on ebay for 6 -7 pounds, or a clip on type for around £25. What's best?

Oh, and finally, any suggestions for the water loss/ reduced pressure would be gratefully received also.

Thank You

Geoff
March 2010