So it looks like all should be fine provided that I can remove the old tails. The question is how. The radiators are old enough that it is likely to need some force. Although the new tails include a square nut section, the existing tails have only a circular outer diameter (again, just like the one shown in the first link). I want to replace these with TRVs ( Screwfix item 32130, http://tinyurl.com/7se3e7p), that have the same 15mm compression connections onto the tail as they have onto the pipe. So they definitely won't work with the existing tails, but they are supplied with their own tails, and it looks like these new tails will fit the radiator.

The Stille reaction is a chemical reaction widely used in organic synthesis. The reaction involves the coupling of two organic groups, one of which is carried as an organotin compound (also known as organostannanes). A variety of organic electrophiles provide the other coupling partner. The Stille reaction is one of many palladium-catalyzed coupling reactions. [1] [2] [3] R 1 − Sn ( Alkyl ) 3 + R 2 − X → ligand set Pd 0 (catalytic) R 1 − R 2 ⏞ c o u p l e d p r o d u c t + X − Sn ( Alkyl ) 3 {\displaystyle {\color {Blue}{\ce {R I have some old rads with tails and valves that look extremely similar to what's shown in this photo: http://tinyurl.com/8aa9bvj i.e. there is a standard 15mm compression joint onto the pipe, but the connection to the radiator tail is rather larger, I think 3/4".

A suggestion I've seen (if I understand it right) is as follows: tighten the tail's cap nut hard onto the valve, then remove the valve from the pipe, and then continue to turn the cap nut as if to tighten the tail onto the valve (except that the valve is now free to rotate) and this will undo the tail from the radiator. At this stage, I removed some crud from around the join - that I should obviously have done before, though who's to say whether the other methods would have worked if I'd done it earlier. I then sawed the nut off and tried to unscrew the tail with a stillson. The outside of the tail was circular so there was nothing for the stillson to grip against, so even when done up quite tight it just slipped round. Finally I drilled laterally through the old tail, so as to insert a large screw or nail (the piece of metal I actually used was a bit nondescript) so that the stillson would have something to grip against, and this time managed to unscrew the tail surprisingly easily. I first tried to remove the tail by the overtighten-onto-the-valve method. Even when I'd done it up quite hard (with stillsons) and then tried turning the nut with the valve free to rotate, the nut still just turned against the tail rather than unscrewing it from the rad. I then tried tightening it even harder, and this succeeded in starting to split the nut, but still didn't get the tail out.