Then I googled it, the computer said that it's 99% sure that one nut had been left somewhere, so back the tractor and I carefully checked that there wasn't a single nut left holding it down, then - it hit me. There wasn't 1 nut but 6!
I hadn't twigged that the injector studs were also head studs, what a plonker

I quickly removed these nuts and then the head lifted straight off, fortunately I don't seem to have done any damage.
Regards
Ian