Have given it so much welly. Four different people’s welly in fact.
Have sprayed some GT85 on it now but I reckon it was over-tightened originally (a friend put it on with a torque wrench).
Failing that I will take it to a bike shop tomorrow.
The good news is I managed to get the chain off so once the fucking cassette is off I should be okay? You should see my flat now, looks like a bike mechanic’s den.