Baxi Combi 133HE - losing pressure + noisy hot water

My boiler (a three year old Baxi Combi 133HE Plus) is making an awful noise when hot water is demanded. especially if the heating is off, but does make the whining/grinding noise even when the heating is on. I've found that filling the system slightly alleviates the noise, but it doesn't go entirely. The heating is working fine.

The system's pressure is also extremely variable from nothing when it's off (is this normal for a pressurised system?) to up in the red when the central heatin is on. One minute you can look at the dial and it might be low or normal, but this can change in a few minutes to being too high or too low - it's all over the place! I have noticed that water occasionally drips outside from a valve (pressure relief - I'm not sure?). I've tested it to see how much and it can leak up to an inch in depth of a small washing up bowl over a couple of days.

Our gas chap recently serviced the boiler and said that there's nothing wrong with it - but clearly there is!!

Can anyone help - yes, we've got hot water and central heating, but the water is noisy and I'm fed up of constantly having to do small fill-ups to restore the pressure.

I'd be very grateful if anyone could shed any light!!

Thanks!!

See FAQ's regarding checking the expansion vessel pressure.

Thanks for the advice. It seems that we need a new expansion vessel .

Roughly how much should this cost to have fitted? And is it something that my husband (who's very 'handy' but not a gas engineer!) can do?

If the cost is over £170 might it be worth calling British Gas, as that's how much they've quoted for a one off repair?

Thanks