hot water issues!

Hi,
I Have lived in my current house since 2003 with a boliermate II and it has always worked very well!
Recently we have been having issues with the hot water. When you run a bath for example the water starts off ok but soon goes tepid - so much so that the bath will be too cold without even running the cold tap!
In the past I have always adjusted the dial on the front to summer and winter during the appropriate seasons with no issues, now however I find I have to have to have it on winter setting to get sufficient hot water even though the central heatig is not back on yet after the summer. Would ther be a thermostat inside the system that is maybe faulty or do you have any other suggestions?
Any help would be much appreciated!
Thanks
Mr F

Hi all,
I notice plenty of people have looked at my post but no repiles yet!
Mike the boilerman - Please can you help?
Mr F

Hi all, me again,

My hot water seems to be ok now i have turned the thermostat to max!
Now the pump seems to run all the time and the green neon light never goes out even if you turn the knob right down!
I am thinking that the thermostat is at fault for this. Can you tell me if this is the right part and how easy is it to fit?
Hawco Capillary Thermostat Pt No XB117 @ £44.39 (Does this price appear ro be about right?)

Please help!!!!!!!!!!!!!!!!