"JWBH" wrote in message
...
We have small two storey terraced house in north London and the gas fired
central heating radiator started leaking in the first floor bedroom at the
tap. When it was opened up a bit the other day.

The nut on the top (not the two flat sided tap spindle) will not take a 13
mm spanner and the 14mm spanner slips around. Is there a special size
spanner I should buy, or do I have to go to town with the stilson?

Try an adjustable spanner, but buy a good one not some cheap far east
rubbish. Best buy is Bacco, expensive but nearly as good as a "proper"
spanner.

Being a complete novice, I'm not sure what my next move should be...try to
tighten it? or undo it and replace. If so where should I be turning off
the water? If I turn off the water and then undo it, am I likely to get
much of a sudden flood? Any advice to a complete beginner much
appreciated, thanks.

If you crank down the valve you'll "probably be ok" but if it all goes Pete
Tong, you'll get all the water in the central heating system above the point
at which you cause the leak. If it was me, I'd try tightening it up but
failing that I'd drain down the system, and replace the whole valve. Use a
thermostatic if there isn't one there already.

A plumber will charge you silly money for a relatively simple procedure.