Well, I've hit it, heated it and covered it with penetrating oil, but it's not moving, till today!!!
It seams that you need a big puller to pull a pulley and this is the mother of all pulley pullers.
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My thanks to everyone for their suggestions. Bottom line is the you have to patient and pull harder than the pulley.
On further investigation it appears that the previous owner has fitted a pulley from another mower which is actually the wrong size, which would explain why the belts kept coming off. So tomorrow I start the rebuild and look forward to mowing again.