I bought a seal/piston kit (new old stock off ebay) just in case.
It is still available from Suzuki.
I took the cylinder off and the piston was stuck at the end of the stroke.
A small amount of corrosion had built up in the bore of the cylinder behind the dust boot, just enough that the return spring inside the cylinder was not able return the piston.
In service the pressure of of the brake fluid was able to overcome this slight resistance.
Put the cylinder in a vice, poked a small rod into the hole the brake hose banjo-bolt goes into, with the intention of trying to hammer the piston out, and the piston shot out the cylinder like a scared cat, hitting a work collage on the back.
5 minutes later we had found all the bits that had scattered themselves all over the floor.
They got a good clean.
Very carful use of a mild acid removed the corrosion, and a little silicone (rubber safe) grease did the trick.
Full function now restored the brakes bled easily.