I've got a Flomaster Triton unvented cylinder and it appears that the
membrane in the expansion vessel has ruptured as water comes out of
the pressure valve on top if I press the central pin.

I've found a replacement 12L expansion vessel here

http://www.advancedwater.co.uk/spare...LOWMASTER.html

but can't seem to find any instructions covering the replacement -
most instructions are for expansion vessels connected to a different
model of flomaster cylinder by a flexible hose from a connector on the
side of the cylinder, whereas the expansion vessel on mine is
connected directly to the top of the cylinder with the expansion
vessel very closely coupled to the cylinder - see the picture tab in
the link above

Does anyone know the procedure for removing and replacing the
expansion vessel on this type of cylinder?

Thanks

Perry