I guess we'll never get to the bottom of it... Just went out and restarted the car and it's fine, ticking over nice and steady around the 900-1000 rpm mark.
Never had issues with damp before on this car - my old Viva was constantly smelling of DW40 because the HT leads were covered in it
I'll keep an eye on things, but I think its all good now - thanks for the suggestions though... I was thinking the worst yesterday !!