A follow up on the Phonak Naida filters: HAB posted them to me very quickly, installation is straightforward (if micro-fiddly!) and the difference is enormous! Really, really worthwhile. I’m grateful for Zebras’s tip.
I used a needle, and took care not to poke straight into the hole that leads to the mic. Anyone else thinking to do it, I suggest practice on the less-critical battery cover first. The Short filters are the ones to get.