How can i change the valve in my Vokera Excell 80SP combi-boiler?

I have a Vokera Excell 80SP combi-boiler in my house. We have it serviced every 6months and about the time we have it serviced we get a problem with the hot water. It either fluctuates between hot and cold or it goes warm then cold. I've noticed that the temp of the water has dropped to 20-30 degrees on the dial meaning there is obviously something wrong. the central heating is working fine. pilot light on. system pressure is up between 2 and 3 bar. don't know how to depressurise only pressurise system. The problems i've had have tended to be the valve or washer perrishing that moves up and down that controls the hot water supply. i've just had a new hot water control dial as that packed in last time so i'm thinking it's just the valve/washer again. The boiler engineer told me that it is a very simple job to do and the part is easily found and cheap. could anyone out there talk me through removing and/or installing this said valve/washer. i feel as though i am capable of such a task but having very limited experiance in combi-boiler maintenance i thought i would try and gain advice from someone in the know. Much appreciated if you could help. Thank you.

Allan Warnock
October 2005