I just finished upgrading and adding radiators, but had one leaky joint.

I was worried about it, as there was a yorkshire tee, with a very short
stub to an obtuse elbow endfeed, giving 5 joints close together to be
soldered at once, or if not at once maybe there's a danger of messing up
the other part of the branch; a horizontal section of the tee failed.

Bugrit.

I can't use a bend, the radius is too tight - the situation is a spur up to
a radiator from a horizontal run that doesn't line up vertically.

Can anyone advise me on the best way to tackle this so I don't fail again?

Should I make the angled spur, and then sweat it on to the horizontal pipe,
or will this endanger joints already soldered, or should I assemble the
whole lot and solder it all up in one hit, which is what I've just done?
If so, is there any order, should I start at the top and work down, or at
the bottom and work up - about 3 inches will cover the whole assembly?

I'd be grateful for any advice to help me get it right this time

mike r