Well, long story short, brake pedal slowly sinking to floor. loosing loads of fluid.

Couldn't spot leak. that'll be the master cylinder seals gone me thinks. Off to the garage with you (daily driver, cant risk stripping it in the street and finding I need parts I cant now drive to collect).

Turns out the air hose I fitted from snorkel to air box has spent the last six months vibrating away nicely and had worn a hole in the down pipe below the master cylinder! DOH!