I did get a reply from Jon (jono) and he confirmed that after fitting the breather modifications, he said he still had some fuel leaks from the filler cap.
I quote his comments:-
?Yes, I completed that mod but my intended purpose was to stop fuel smells - it has achieved that. I still get a petrol smell in the boot but I think this is due to the connection hose being vapour permeable.
I still get fuel leaks from the cap and would like to get this sorted. Mike Duff pointed out that there is an inner seal in the fuel cap which you will find if you take the roll pin out and remove the main seal and retaining plate. He replaced the secondary seal with a thicker one and he reckoned it eliminated the issue. I tried the same mod and it certainly improved things greatly if not completely eliminating the problem.
The Mike Duff cure has, I think, potential to cure the issue but I never sourced the best size of inner seal which I aim to do this year in an effort to cure this once and for all. The MX5 mod is certainly interesting and I may try that as well.?
Jon?s comments adds to those made above by Alan and Dougal.
So at my stage of restoration, I will incorporate both the Mike Duff cure and the MX5 mod