It turned out that the bleeds are a tiny 6mm. Fine when new, but they will be a pig after a couple of years unless kept nice and greased. Worth noting if you buy refurbished calipers.
Notwithstanding this, I've finally admitted defeat on this job. With the new caliper fitted there was unsurprisingly no brake pedal. However, when we loosened off the bleeds and pumped the pedal we got nothing, not even a single drip of fluid, and same for the other side.
Any ideas as to what the problem might be? I'm not going to tackle this further myself but it would be interesting to hear what the problem might be. I think issues like this have been posted before and a pressurizing tool was the answer?