Just got round to bleeding the brakes on my Visa, then with very little pressure on the spanner, the front nipple sheared off flush. no chance of getting it out !!!...
So I picked a spare BX rear one I had in my box of bits that had a similar fate. and set about using this as a test bed...
I carefully drilled the remains of the broken nipple out in stages, to try and eliminate the drill running off centre, this worked, and I tapped the new hole 8 x 1.25..
I then got some 8mm screwed rod and fabricated a new nipple, using just a drill, files and hacksaw blades.
Drilling the hole down the rod, and keeping it near the centre was quite tricky, but managed it.
Cheers
Pictures below.
Now I will have a go on the Visa caliper...

The End Result - Showing part of the old, and the new nipple

Forming Another Bleed Nipple - Action Shot