i asked this question earlier and 'john' gave me an answer.
john if you're reading this would you take the time to quickly go back to my initial question and see what i wrote, saves me having to re-write it all again?

chris gray, February 2010

well come home to a nice hot house and fully functional boiler. i now have a expansion vessel on the floor attached to the flow side of my c/h, its not the bonniest of things but at least it works.
and for £50 which is what hes charged me we're both winners.

ive just reread my whole dilemma and think that this all may be of use to someone else in the future. chances are they wont read this far back and it will be lost to the archives.
cheers and thanks again john

chris gray in his heated home, March 2010

yeah i got a plumber tomorrow, hes going to just stick another vessel on below the combi, as the worcester one is at the back and a chunk of work to swap over.
thanks for your advice mate.
i wired his en-suite up sometime ago and did it for mates rates, so hope he's going to return the favour when it comes to payment.

chris gray chanting 'british jobs for british workers', February 2010

well done mate, you have self diagnosed, best to let the experts change the vessel, its a rotten job, worcester offer a prompt and reasonable repair service and will give you three major parts for a fixed fee, so if other things go wrong during the repair they have parts on board, also they will change the auto air vent and prv while there as the engineers loose money if they have , recalls

john, February 2010

cheers john
emptied the vessel, pumped some air into it, repressurised back upto 1.2bar, and called for c/h, the gauge slowly goes up. turned it off and left it last nite.
went to it this morning and the gauge had fallen to .4 bar and there was no air in the vessel.
going to have a new vessel fitted tomorrow.

chris gray stalking john, February 2010

the amount of pressure in the vessel isnt critical, if you push the tyre pin and it hisses and you have pumped it and expelled any water that may have been within that is somewhere near good enough. if your pressure gauge isnt working now that suggests to me dirt or sludge has blocked the pin hole within the nut that connects the copper wire between the gauge and boiler, if you totally drain the system and undo that nut and try to clear you have a chance of reusing it, there is only so much advice to be given in a situation where the boiler cant be seen, if your system is heavily sludged then that may be the cause of all the problems up to now and will require flushing and cleaning.as an after thought are you sure the point where you are filling the boiler isnt continuing to let by once you think it has turned off and causing the boiler pressure to go too high? disconnect once filled if unsure to inspect. good luck, i hope this helps

john, February 2010