The message .com
from "Fitz" contains these words:

This time I closed the lock valve, applied pressure to the nut to turn,
bracing the pipe as usual and a small jet of water suddenly started
shotting out from the compression joint securing the lock valve to the
supply pipe.

One possibility is that the pipe was cut off a bit short and the olive
is squeezing down on the end rather than biting into the pipe a short
distance from the end. I am not saying that is necessarily the case but
it fits with the fitting starting to leak badly as soon as you started
to mess with it but if that is the case there is now a faint possibility
that the pressure will push the valve right off the pipe now you haven't
got a radiator holding it in place.

Given that the radiator position is fixed the only cure (unless you are
very lucky and do have some slack) would be to cut back the too short
pipe and fit a slightly longer length. Don't go to the other extreme
though. If the end of the pipe is hard up against the valve before the
nut is tightened then much of the force exerted on the nut will be
dissipated in forcing the olive along the pipe rather than into it.

--
Roger