SUCCESS.
had a 90* chuck on my drill (went through 3 of them actually, dont buy the one in screwfix, the gearbox is plastic) and managed to drill up to 9.5mm. then i undid the head bolts that would undo and got brutal with it, levering the head off the block till the bolt snapped.... which it did quite easily.
now ive just got to have the heads pressure tested, skimmed and ported and polished (why not whilst theyre off) and stick it all back together
and the reason for the blown head gasket?
afore mentioned numpty previous mechanic decided to torque it down when one of the dowells wasnt aligned with the hole in the head. so that cyl head wasnt sitting square. idiot.