Today was going to be engine out day and I set to disconnecting everything. One of the exhaust down pipe to manifold studs was very tight so I sprayed it with penetrationg oil and set about removing the bell housing bolts.
That's when I saw the problem. The clutch operating arm has two adjustments. The bottom one adjusts the free play and the top one is an adjustable stop. The adjustable stop had adjusted itself and was hitting the bell housing bolt before the clutch could release.
I moved the adjustment and reassembled all but the radiator & bonnet, fired the old girl up and hey presto! the clutch worked. Just the reassembly to finish off now and we should be good to go.
A lot of work for something I should have thought about before I started pulling things apart.