Not the greatest picture - this was taken before removal.
The inlet nylon elbow on the vaporiser was shot to bits. Literally crumbled away once I had the unit out.
I bought a refurb kit
PRINS VSI Reducer Repair Kit (B,C,D,E) - my reducer had E4 on the label inside a circle.
from
LPG Shop - Autogas Parts, Kits, Accessories and Conversion Equipment
I also rang them up to make sure I could disconnect the gas (there is a solenoid on the tank so it is okay to undo the gas pipe to the vaporiser.
Note, if you do have the same situation the new rubber disc looks totally different from what you remove (they were the same diameter). I checked with lpgshop and the deformation was simply due to internal pressures.
Note the rubber disc was not the cause of the leak, it was purely from the inlet elbow.
Charlie