Anyhoo...
I decided to remove the rear subframe today...
Squirted all the nuts and bolts liberally with WD40 yesterday and hoped for the best...
Removed all of the bolts apart from the two front fixings on the drivers side and then took the dog for a walk... Rather stupidly, when I returned I thought "it is going so well I may as well just remove the last two bolts..."
The first one sheared straight off as soon as I tried to turn it...
The second came halfway out then the fixing in the sill snapped off leaving the bolt half-in and half-out with no chance of removing it or the subframe or getting a grinder or a hacksaw on it... So I had to drill out the head of the bolt then chop the top off with a chisel... Two hours later the subframe is out...
The offending bolts...