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?

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.