Hi sorry Pete - here is update.
So ordered a cheap version of Techstream from E-bay seller in Bulgaria (£27 for disc and cable) My local garage didn't have software. Once programme loaded on laptop it can connect to car ECU's. This allows you to disable EBC and so bleed front front brakes in usual way. Removed old caliper and go to mount refurbished caliper to find mounting holes don't line up (about 5mm out) Everything else on the caliper is identical. However, there is a 2 digit code stamped in to caliper (mine was N18) and the one from USA was K7. Clearly this made a difference. Damn! Plan B.
Removed pistons and seals from re-con unit and fitted them in to my old caliper. Old pistons are plastic (Phenolic) - replacement are metal. Apparently Phenolic pistons are used as they don't transfer as much heat from pad to brake fluid. However, internet search has reports of these plastic pistons absorbing water from old brake fluid which causes them to become oval shaped and so stick. So maybe this was cause of my problem as the piston bore was smooth despite split seal and the plastic can't corrode.
Soo - all back together, bleed brakes (even with ignition off there was sound of a motor/pump from engine bay when brake pedal pressed) The Techstream software explained that once ignition switched back on, the EBC was enabled. Went for test drive, no warning lights and brakes feeling much better.
Am planning to source seals and pistons for when passenger side starts playing up.
So - bleeding fronts is easy with the Techstream. Rears are a different matter though. Any questions, I'll try and help.