Cheers Guys, have had a look.
The rear threaded bit on the calliper is about 15mm shorter on the rear than the front. I can strip it down and swap over the threaded post, but given I don't know what caused the front failure despite stripping the calliper back to it's component parts looks like replacements are in order.
For the moment I have used an old silver Tektro caliper as a stop gap. Doesn't match the black Sram rear but will do in the short term.
Pete if you come across any decent secondhand ones on your travels pm me.