thanks all for the suggestions.
yes, it may well be an expensive option to change a tube, but out of 15 frames i’ve owned over the last 4 years this one just feels like it ‘fits’ much better than the rest.
for that reason i think it’s worth at least asking the professionals for a quote/ opinion.
taxi – i noticed the shim, and your suggestion did cross my mind, but i suspected there might probably be some structural reason for its inclusion.
i have tried pure racing and gravity droppers, but wasn’t overly impressed, and that’s when i came up with this hair brained scheme.
i’ll reveal the cost once enigma get back to me.
cheers, stan.