Sime super 120 de luxe - high pressure problem?

Hi, I have read a few similar questions on this site, so appologies if this is not new, but here we go..

The pressure on my Sime super 120 de luxe goes from 1 bar to 3 bar when the heating is switched on - and then water leaks from the safety valve. When the heating is switched off, the pressure returns to 1 bar (ok, a bit less than that due to the lost water). Replies to similar questions have suggested a leaky heat enchanger leaking mains water into the central heating system, but i'm not sure this makes sense in this case as the pressure drops after the heating is turned off...

Two other clues:
- The pressure problem has gradually been getting worse over a few weeks, but it has only just started to hit the 3 bar level and start leaking.
- I think the start of this problem coincides with the point at which the temperature gauge stopped working - or is that just coincidence?

Any advice on how to fix this please?
Thanks,
Mark.
(01/01/2007)