Aviation Herald sez "crew reported smoke in the cockpit about 140nm northwest of London (almost over Liverpool) and returned to London Heathrow for a safe landing on Heathrow's runway 27R. Responding emergency services found no trace of fire or heat. The passengers disembarked normally."
I guess it can't have been very smoky or they'd have put it down quicker (Manchester?). Maybe we'll get an AAIB report in due course.