Thanks to all for their help. Plumbase don't keep it anymore - first stop that was ;o))
A quick blat across town this afternoon on the XBR - so good at herding traffic - to Cromwells. There I paid less than a tenner for 500mls of Baker's No3.
As it is in a plastic bottle & not a tin, it won't silently rot the tin out & soak into the floor in a pile of white crystals. This to be discovered at
7.30pm on a Sunday when you come in from a ride with no front brake & work tomorrow. Ensues a battle with recalcitrant Bowden inner, Fluxite and solder wire resulting in a nasty mess which you promptly do again when a new tin of Baker's arrives!
500mls might well see me out!