So, has it made any difference so far?
In a word: YES!
Little Willie now pulls like a thoroughbred racehorse and revs extremely smoothly at the merest touch of the twistgrip. It sounds totally different too; no hint of the 'woolliness' I'd become used to; a much sharper exhaust note.
Normally, when moving off from my house, by the time I get to the end of the road 200 yards away I've just about got up to 30mph. Now, within a few yards, I'm up to 30, and by the time I reach to end of the road I'm doing a good 40mph. A neighbour saw me - but failed to recognise me! - and later said had I bought a new bike as it sounded so different!
When I removed the Amal, I also took off the twistgrip and throttle cable assemby. I made up a new throttle cable using a brand-new cycle cable from my local bicycle shop - price £3! It came complete with the necessary fittings and was a doddle to set up (and solder). That's quite a saving on the normal £9.95 throttle cable bought online, isn't it? And then there's post and packing to consider as well! Why not try making one up yourself? You'll be glad that you did - and it'll save you a pretty bob or two into the bargain!
PS: I've just stripped and checked the Amal (the carb, not me!), and it seems the slide is worn oval and pretty-well knackered!