I had exactly the same problem when I first got my GT6. When closing the fuel flap the rubber seal was not pressing tightly against the petrol inlet pipe. I have improvised a spacer behind the seal as a temporary fix until I can work out a permanent solution.
Not sure if there is a cheaper way than paying for a "Heritage" certificate - but I am sure some one else will know.
https://www.britishmotormuseum.co.uk/archive/heritage-certificates