Thanks all for your help.
Moke Spider was correct with the rubber grease theory, which seemed to have blocked the line between the master and the bias valve (26 in that diagram I posted). I replaced the line between the two and bled the master cylinder and that valve again.
Unfortunately a day after I noticed a puddle of fluid under the rear passenger wheel - the rear cylinder was leaking profusely into the drum so have just replaced that this weekend. Whole system bled with pedal pressure holding now. Life wasn't made easy due to all the brake lines and unions having underseal painted all over them, but thankfully no rounded unions.
Appreciate all the tips!
