Glowworm boiler high pressure

I have a Glow worm fuelsaver boiler circa 13 years old. The pressure range whilst it is fired is increasing, to the point now where from being at less than 0.5 bar when cold to almost 4.0 bar after about 1 hour.

There now seems to be a rattle / ticking noise when the boiler first fires also.

At present I am only running it for short periods of time to heat water.

I have checked the filling loop to make sure it is disconected with no pressure from the filling pipe.

1 the prv should open at 3 bar THIS IS DANGEROUS

2 the expansion vessel may be under pressure