Is it cranking at a good speed or on the slow wheezy side when it turns over?
If the latter then it may well be down to poor ground connections assuming the battery is in good condition.
Also, not that it should make a difference to it starting but I am pretty sure Shell Nitro (and their V-Power) contains ethanol which does all kind of horrible things to your fuel system (and plastic fuel tank if you have one).
There's an odd tolerance allowed in the actual stated percentage of ethanol allowed in fuel at the pumps in the U.K. (in most of Europe E10 (10%) is common place) but the only one I know for sure here that doesn't have any is BP's super unleaded which is what most of the classic racers use (particularly those running two strokes as the ethanol also effects mixture).
http://www.telegraph.co.uk/motoring/...and-bikes.html