Well good old Tony at the brake shop came through anyway. I was a little bit down on him the other day when the caliper leaked, but he has come up trumps. Not only did he manage to get another seal kit when most brake businesses are closed, but he also found me a litre of silicone brake fluid. And drove and got it for me himself. And sold it to me at cost.
Caliper is now working and installed and... I have brakes! Well I have a pedal anyway. As the car is currently up on stands with no wheels it might be premature to assume too much.