OP he thanks for all the replies, people: very instructive as ever.

I will have a look at the plumber's torch mentioned by one poster, and
also at the Nitromors "mousse" which looks less evil than traditional
Nitromors. I will also look at making my own gel (I actually have some
caustic soda in the garage! (It's next to my can of creosote.))

Most of all I like the idea of the torch with hose and gas tank, but
that would be overkill for what I'll be doing (two large frames only. I
hope.)

Of course, there's the other question, which hangs at the back of the
mind these days (into my 50s): will I ever need to do this again?!

Cheers Chaps
John