The alloy cylinder reacts with the cast bellhousing causing it to seize. I’d suggest using a proper penetrating oil (not WD40) and allowing, say, a day for it to work. Freeing is then a matter of twisting and hammering until it works free.
The rear main seal is a rope type on the pre-crossflow engine and requires the sump’s removal and change of sump gasket too. Although it is possible to do this without removal of the engine, it is very awkward and the confined space makes mistakes more likely, resulting in further leaks. But I’d check that the sump bolts are all tight before doing anything else, and that the breathers are clear.
I had a feeling that was the case on the cylinder. would a little heat be of any help as the slave cylinder is going to be rebuilt or replaced anyway
Its not a big job to get the engine out (compared to some other cars I have owned) and as I need to take the gearbox out to do the clutch and some other bits ill drop them both out. Its always so much easier to work on something in front of you than above your head.