For anyone else considering this I thought I'd post my experience.

I had a sagging Swallow which was running out of tension adjustment - it had stretched that much! I bought it secondhand so I've no idea of it's history but it was very (too) pliant.

I boiled it for 2 minutes and it did indeed thicken and shrink. However I stupidly removed the tension bolt and getting that back in place was a real pig of a job. So my advice would be to loosen it right off but don't remove.

Several applications of clear shoe-polish later the saddle looks great, is the perfect shape, and is rock hard - like a new Team Professional. I'll now start breaking it in, adding proofhide as necessary.