OK, I'm no plumber but I had a similar problem with a Sime Super 90 Plus. It was dodgy when we bought the house (we didn't know) and it got progressively worse.
The problem was the 'Exchanger' - it was corroded beyond belief - ot was so bad that slowly water leaked from it, bringing the pressure right down. Eventually we were at the stage where everytime I topped the pressure back up, the Exchanger would leak worse than the time before.
This therotically is a time bomb, waiting for it to go 'bang' with a fair force.....
The solution was to get a new Exchanger - this cost us about £100 plus labour.
I suspect that your problem is the same, but your in the early stages at the moment. The Exchanger looks like a small tank when you take the front cover off.
Hope this helps, but I may be completely wrong....
Gavin Sever
May 2005
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