Well, I managed to get injector no.4 out of the cylinder
I thought I'd share how I approached it, in case it helps someone in the future.
-- Continual soaking with diesel didn't seem to do anything, so instead I went with a healthy dose of crack it freeze spray on the injector at the top, immediately followed by a dose of the penetrating version of WD-40 - theory being it might have just enabled a little bit of lubricant down.
-- Quickly upgraded from a standard 15mm spanner to a large 12 inch adjustable spanner which sat around the injector more securely.
-- Purchased a "spanner extender bar" from Amazon - had no idea they existed, but best £30 I've spent in some time. Enables you to get more torque on a spanner
-- Also purchased a cheapy injector puller from Amazon for £20.
Releasing the injector consisted of soaking, then very gentle but consistent clockwise torque to begin with, then soak, then turn. Then heavy soaking and leaving alone.
Eventually the injector started to rotate fairly consistently, and after about an hour at it, I noticed at one point a slight change in the amount of resistance. I was desperate, so I gave it a little bit of anti-clockwise motion, then clockwise again and then there was a little bit of easy movement - hey presto, injector puller on and out it came.
I suspect I spent 5 hours on this, 3 hours spread out from my original post, then 2 hours last night. Ironically, the injector puller, while useful at the end, I could have probably managed without - the spanner extender was the winner here. And I did this with the cylinder head still on the block, there's no way I could have secured the cylinder head and applied as much torque as I was initially without it still being bolted on.
Photo of the little darling, the tools used, and my celebratory beer afterwards