potterton combi boiler pressure

Hi all,
I have a problem and think here is the best place to get info...
I have a potterton combi and recently it started playing up, when I run the heating from the boiler it runs ok at first with the pressure reading at 1.25 bar, then over around 30 mins the pressure increases and increases itself and then eventually when it reaches around 3.5 bar (ish) it shutsdown and then we have to allow the pressure to drop before we can reset it and then its the same again, British Gas attended and has said in their report that it needs a PRV outlet to be fitted, but its been running as it was for 8 years without any faults, does anyone know what it may be?
Thanks in advance guys

expansion vessel needs recharging

drain boiler

put pump on schrader valve on expansion vessel

pump up to 1 bar approx

refill

was in faq but has been removed for some reason

Or it needs replacing. In which case it will need to come off the wall if it's an Instant.......you might get away with removing the flue and easing it out if you have enough height clearance. But try recharching it first (boiler cold and empty, pump up the vessel to 1 bar and then refill boiler)

If the expansion vessel has gone, screw taking it off the wall. just cap off the outlet for the internal vessel and fit an external one on the return as close to the boiler as possible. Normally combi's come with an 8 litre vessel but i always allow a little more by putting a 12 litre in. Allows for any future system extensions etc

superb guys thanks very much