Well, I weighed up the options regarding my Avid brakes, and I chose the cheaper/short term solution.
Ordered 2 internal rebuild kits and a bleed kit.
Taking apart the Avid brakes was fairly straight forward, as was putting them back together, although I now understand why they aren't rated very highly. The piston is only held in with a fair unsubstantial plastic circlip. If they fail again in the future I am definitely going Shimano XT.
The bleed process wasn't as bad as I thought, the rear was easy to do and worked fine afterwards, but I had to do the front 3 times, I mistook a lot of air coming out of the caliper as air being sucked through a seal somewhere, when in fact it was actually air in the caliper, D'OH!
So on the third bleed I made sure there was no air at all coming back out of the calipers when using the syringes.
I would not like to do more than a couple of those per day! I wonder how professional bike mechanics think of the Avid bleed process.
Anyway, all back up and running perfectly!