I think I have a handle on it now although not a way of fixing the problem. Thanks guys for your input it has helped me see the wood amongst the trees.
It seems that when the rear brake gets hot due to prolonged braking down hill ( my experience ) or if the handbrake has been left partially applied, the drum expands allowing the auto adjuster to go one notch further than normal - it is a very fine ratchet adjuster. This keeps the brake applied generating more heat - gassing off any moisture in the fluid and giving rise to the pedal going to the floor. The only remedy seems to be removing the back wheel, levering the over-tight shoes and brake plate out of the drum, re setting the adjuster and putting it all back together. Change the fluid just in case it has deteriorated and carry on as normal. Short of changing the pitch of the adjuster ratchet I see no obvious way of avoiding this annoying and possibly dangerous glitch from recurring.
