Last night I turned off the rad valve in preparation to remove the rad for
decorating the room. Once I turned off the manual valve it started leaking.
Basically, the valve itself has sheered off. I had to drain the entire CH
system last night to stop the water.

Question is, is it possible to buy just the valve or do I need to buy the
entire rad valve inc the housing?

If I want to replace the manual valve with a TRV, is it a direct
replacement, or do I need to remove the tail from the rad?

Thanks