Thanks Soundguruman - I was lucky. I had previously worked a little WD40 into the sleeve. After an hour off task I tried a small screwdriver between the spindle shoulder and the brass bearing sleeve and gently prised it and it moved and I gradually eased it apart. Previously I'd been trying to rotate it. As other posts have said there was a residue of white solid material at the top edge and this had caused it to seize up.
What I hadn't realised is that the grease in the brass sleeve presents a lot of suction so pulling the spindle shaft out requires some steady pulling or it sucks it back into the sleeve, like a piston in a cylinder.
Cleaned all the old grease off and relubed with a little "car grease" and it now works perfectly.
Thanks again for the tip. I was lucky not to need to do this, but anyone else trying the screwdriver method it did start to move quite easily - I had been concerned that I would chew up the brass in some way if it hadn't moved easily.