Luddite , Mr Gazza- Thanks for the comments/suggestions. I saw them after I had started this morning.
Using virtually the last bit of my road sign I knocked this up this morning using a Schrader valve stem off an old inner tube.
photo sharing
There is a 6mm thick closed cell rubber gasket between the filler neck and the circular plate, the gasket avoids me having to seal the filler recess drain pipe which runs right through the tank exiting next to the fuel tap.
The tank breather pipe will be connected to the one way valve and if this leaks a 6mm bolt will be threaded into the breather pipe and clamped with a hose clip.
The more complicated end was the fuel tap boss which has 2 x 9mm holes drilled very close together to accommodate the on and reserve portions of the petrol tap. I could only tap them out 10mm but even this was not satisfactory as the correct drill for a 10mm x 1.5 is 8.5mm. I tapped the holes anyway but the resulting thread was almost certain to be porous. Then I had a brain wave, just install the tap which should be petrol tight and therefore very nearly air tight.
My plan is to wait about 5 days until all the ethanol resistant resin has fully cured then put about 15psi into the tank and see if there is any air leaks, if there are none I will leave it overnight and see whether there is any appreciable drop in pressure. I am not sure what I will do if there is????
The rest of today was spent cutting the closed cell rubber foam for the seat base, first a cloth pattern was cut (top tip do not use cloth that has a capability to stretch as I almost cut the first sheet of 6mm foam too small). Then this 6mm layer was trimmed to fit over the whole base (top and sides) then a 12mm layer on the top surface only. I had intended to put another layer on but I do not think it is necessary as pressing with your finger you cannot feel the CF base underneath. The ambla was then roughly cut to size ready for trimming tomorrow or whenever my longer csk screws arrive.