With a bit of sleuthing I worked out the the 200k Ohm thermistor in the extruder nozzle was drifting out of tolerance in a big way.
A quick email off to BFB and Michael kindly sent me out four spare thermistors.
Let the rebuilding begin!
The extruder heater, nozzle and thermistor assembly are all held together by fire cement, this necessitated a little exhumation of the nozzle :)
I soaked the nozzle in hot water to soften the fire cement (I more than once very nearly took a swig of cementy water thinking it was coffee...)
After a little persuasion the cement was removed allowing the replacement of the thermistor :)
The printer has just this minute successfully completed a test print :D More to come.