It is out! Turned a Snap On square extractor into a Snap-Off square extractor. Ended up taking the head off (scary) and drifting the snapped extractor out and then drilled the fucker to within 0.25mm of the threads with an £8 drill mounted on a £6 pillar. (Using a £45 cobalt drillbit set). Then matie started tidying the top end of the remaining metal with some, er, needlenose pliers, and the thing started turning out leaving a perfectly intact inside thread! Result! The motor's all back together with lovely new plugs, ready for refitting tomorrow...