Thanks for the tips, folks - I might check out the zinc thing first of all. My nails had always been pretty good for years. However, since I started back into fingerpicking on .012-.053 gauge strings - again after many, many years on .009 electrics - I've noticed that they're not quite as resilient.

I also have a flawed plectrum technique - I don't tuck my first finger in enough, so its nail wears down when I'm playing consistent rhythm at sessions or in the ceilidh band. I've now overcome this by using a Bumble Bee thumbpick which - unlike others I've tried - gives a clean, crisp sound and is easy to manipulate quickly. Using this for the moment is allowing the nails to grow back again.