Hi all and thanks for the ongoing comments.
Sorry to say I gave up on it for the moment. It's on the shelf in bits looking forlorn. In reply though, david.tamlaght, the threaded rod looks fine. I did take it to the tool shop and the guys couldn't see anything wrong with it either, except for a couple of tiny chips in the knob casting, presumably from when I took it off. I don't know where I would get a spare nut for this kind of thread to test though.
Dee J, I reckoned on a value of around £100 or so if it's in really good nick, which mine would be - if fixed. In current state, it's only good for spare parts! No, the thread isn't punched. To remove the sole and unscrew the nut, you have to tap out two pins that hold the sole on to the body. Once that's done, the thread ought to unscrew freely. Mine got stuck and I had to use some force to get it off, but not what I would have considered excessive, but I reckin that must have done the damage. The thing is that it's impossible to properly see inside the knob to visually inspect the thread because the hole's quite deep. The first couple of turns each end look OK, but it absolutely will not go on upside down and I can't see why not.
My next best plan is to brush on some coarse valve grinding paste and spend the rest of my life trying to polish it back into shape. There aren't any engineers left around here AFAIK, so retapping it would be beyond my capability and probably budget. I'm not sure I even have the skills to make a scraper, but I can have a go...
There does come a point where you think it's not worth the effort, or I could just buy a rougher s/h one for the parts, but it's still annoying to chuck out what ought to be a nice tool!