Thanks Bugsy.
yep - I guessed the thumbpick's mebbe the problem - but it's how I play I'm afraid.
Tried many times to give it up (hah, sounding like a baby with a dummy) - but it's no use.
Thirty five years of hand position won't be retrained in my case.
My thumb is parallel to the strings with a thumbpick - but has to be at more or less a right angle to 'em to play wi my thumbnail.
...and if I use the fleshy side-of-thumb, I dinnae get the attack my own percussive style requires. Grrr - it's all problems innit!

But is the answer to my question, then, that these 'ere folks that say elixirs last 'years' are not using picks or plectrums, but just soft flesh?
Cheers - Duncan