well, to make it easier remove the coilpack (4 bolts, 6 leads, and one plug). all the allen bolts in this area of the engine are very prone to rounding...replace them with new ones from the dealer when you get them off...saves hassle later. i had the one you're talkin about round...in the end i just snapped the housing off and replaced that part of it (£20ish), then angle grinded the head off the bolt (then you can usually just remove the rest of the bolt by hand).