Managed to get three undone, but sheared off the last one. Has anyone experienced this and how did you remove it. Many thanks. Anthony.

 

If you are talking about a bleed nipple on a brake calliper then just forget it and bleed it by loosening the brake pipe fitting

 

Left handed drill bits. If you're lucky it will snag it and back it out. The worst you can do unless you mangle it is drill the hole bigger a step at a time until you can free it. Otherwise you're buying a new caliper. TM

 

Soak it in brake fluid then try an easy out . Otherwise, there are companies that will drill it out then re-tap to take a larger nipple. If you google "brake repair specialists near me " Then get on the bell.
Bet it did not have the rubber cap on it and water got in or fill with grease.

 

If you can’t remove the old bleed nipple, drill out and fit this. See pic.

 

The bad news for you is there are 8 bleed nipples on those brake calipers, not 4.