A propane bottle would give you more capacity, and a valve already
fitted.

Thanks everyone sorry to be so long but its not through disinterest
but through labouring over all possibilities. Thanks Rob re propane
tank, I found one by the kerb as you do, £15 to a welder and Ive 44
litres of tank emptied to 18 inches mercury.
This sucks my vacuum form down fine but there is one spot for
improvement. The plastic is held in a frame over a 18 inch sqr grill.
Frame gets hot and is pushed down onto the vacuum table but the frame
to table seal is not great. Have any vacformers out there a good seal
idea - rubber oven door sealant? It has to take the heat and be fairly
air tight
Evacuated