For reference, I tried fitting these:
https://www.ebay.co.uk/itm/2x-Univer...N/184222724225
And they're the right size, but a bit too strong. You have to put your hand over the strut attachment point when closing the boot floor otherwise the floor flexes before it compresses the strut.
The other problem is that it made the metal bracket on the chassis bend slightly (which caused the ball joint to pop off), so I have to put a couple of bits of wood behind it to stop the bracket from bending. Ideally, I'd have fitted something weaker, but I had these lying about. Sounds like something between 140nm and 240nm might be the sweet spot?