Oh dear, still no good. Heated it locally, penetrating oil on hot bolt so that it might take in some as it cooled. Used impact driver with new socket and adaptor. No effect at all; this might be due to the ~10 yo 12V Makita ID. Chap down the road works on cars on a professional basis (e.g. _complete_ stripdown, overhaul and rebuild of a BFO Range Rover engine). He has air-driven gear for wheel nuts - methinks build up slowly, as an 8mm bolt might shear.
Gave up on trying to remove the blade - too high a risk of shearing the bolt. Used a Powercraft multitool sort of thing - v small chuck that takes tiny heads - and ground the blade in situ. Any final dressing done with a diamond file. Cuts OK, but the gass is now so long that the bag fills up very quickly. Might wait until the grass-cutters come round or, if necessary, use the Bosch brushwwod head. The head is a lot easier than the strimmer head on long grass.
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