I'd quite like to play with a hornet based wildcat in 22, 6mm or 6.5 and heavy match bullets.

I'll only need about 18" of barrel to fit a BSA martini action (so pretty heavy profile)

Tired F class barrels likely to be the best? I was sort of presuming that even when retired the area past the worn throat is generaly pretty good?

I'd start with a polite request to the 'smiths who list on, say, benchrest.com. No doubt they pull a few barrels in a years time and might care to humor you. Next list would be the match 'smiths in the back of Gun Digest Annuals. You might also look up local competitors in your area and ask who does their work. Might find a local man with a barrel full of barrels just sitting there... Might. I have known a couple, with current heating prices, who remember all the mil surplus stocks they put out for the trash man. Good wood. Hard wood. Greasey. Now if they had a wood stove...luck