My bad luck was a stripped spark plug hole. It was extremely tight to remove as you can see from the slippage marks on the flats.
I have 4 SD1 heads, one now has a stripped spark plug hole on cyl #4. I have run a tap through it but really it needs a helicoil. I would say about 1/3 of the threads are OK, 2/3 are not. Trouble is that on close examination neither of my 'spare' heads are much better, both have sustained damage while they have been stored and in frequent moves from one location to another.
At the moment I am staying with the first head with the stripped plug hole. I have tried a plug and it seemed to take a reasonably tightness without seeming to yield.
On a more positive note I finishes assembling and fitting the distributor, which meant I could assemble the oil pump. The Rover V8 has one particular characteristic, the oil pump is set high and dry, not submerged in the sump like many engines, therefore it needs priming before it will pump oil. The standard method is to pack the oil pump with Vaseline which then causes the gears to suck up the oil from the sump on first starting. The Vaseline then dissolves in the oil once the engine is running and up to temp.
Once the engine is fitted things can leap ahead with the build. I hadn't realised what a logjam it had created.