The fekker is out.... decided to drill the head off. It worked.
I can't weld or fabricate stuff from flat sheets of metal so these pics probably won't impress those of you
who build your own cars and ****, but I'm well pleased.
But they might inspire some people to have a go, rather than just take the car to a garage.

Drilled a small 1.5mm pilot hole as near to the centre as poss. then followed with a larger drill bit.

Think I got pretty close to the centre...

As the swarf/shavings built up I used a screwdriver with a magnetised tip to clean them up

When the drill bit broke through the bolt head it twisted a bit out, which I used to drift it with an old! It started to move and after a few turns could be removed by hand

All so I could change the spark plugs.....