Yes it is on a DT tank with a flip-up cap :-)
l realized that a fuel tank cap has to breath so l drilled a very small hole through the post that the sealing washer slides up & down, tapped a thread in the top of the cap and fitted a 5mm screw with a a hole drilled through it also, on this screw (with the head cut off) l can fit breather hose, with an inline fluid check-valve. l sealed around the outside of centre-post so that fuel could not escape via the original breathing passage. This way l have control where fuel might come out.
The original sealing washer around the outer edge of the spring-loaded washer has long since vanished, which started the whole process. l will experiment with a full-coverage rubber seal, held on by the centre circlip, this should be easier than trying to seal just the outer edge of the spring-loaded washer.
Good work if you can decifer my explanation ;-) Thanks again for input, man l am so impressed by the knowledge that is shared on the website.