[Snake2998]

Thanks for the input all - you have given me some pointers to look at.

A couple of replies:

1 The pedal is rock solid as long as there is vacuum - it doesn't sink at all.
2 It is definitely a vacuum booster.
3 I haven't tried the pedal after switching off to see how long it takes to lose vacuum - i.e. to see if the one way valve is working.
4 The pedal only sinks when throttles open with engine running - I haven't tried it when in neutral - will do so next time I get to the car - it's currently 50km away from me.
5 The pedal is rock solid before starting the car after a week or so - haven't tried starting with pedal depressed to see if the pedal sinks a little as normal when vacuum is restored - will do so.
6. There's no apparent fluid loss in system as brake fluid reservoir is still full.