expansion vessel?

i have a potterton lynx2 boiler and have been told my expansion vessel has "collapsed". Due to the type of year it has been hard for me to get hold of any expert.
when i switch the boiler on it no longer fires up. The demand light flickers a couple of times but thats all that happens. the pressure valve was changed two weeks ago but i still had to keep turning on the water inlet tap so the pressure would stay up.
Anyone Know anything bout expansion vessels?? Where can i get one, the cost of one etc etc.