"N. Thornton" wrote
| sounds clever. But... the pipes are cemented in, so no movement
| at all. I'm beginning to think the only option is to drain
| down, saw through the pipes, and reconnect with compression
| fittings afterwards.

There only needs to be enough give to enable the valve to be separated from
the radiator by mm. If they got the radiator in with the pipes cemented,
then you should be able to get it out.

Owain