is this really the best place to conenct
your new rad in to? I'd be inclined to connect back to nearer the main
'trunk' pipework rather than a branch

Yes, it is the main trunk, not a branch.

And I'm getting pretty good at the old solder joints now, but the
comments about water in the pipe is well worth noting - I hadn't
thought of that. So I'll do the link to the exiting pipework with brass
compression joints.

Thanks again all
-K