Thanks for all the input on this subject. Opinions were a little varied but the general consensus is that I am indeed wasting my time trying to get the Gran Sport rear mech to work with a 28 tooth freewheel cog and I should go and buy a Shimano index system instead :)
This valuable info means that the next time I have a spare half an hour to spend tinkering with a bike, I'll be able to do something genuinely worthwhile instead of adding and subtracting chain links in vain.
Thanks, Bruce
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Bruce Robbins
Near the hamlet of Lundie
in the gloriously green and
rolling Angus countryside