Hi Robin;
I had this challenge last Spring.
The boatyard lifted my boat on a cradle so that I could lower the keel and antifoul.  It got stuck about half-way down and no amount of banging and brute force could shift it.  It transpired that a ‘bolt’ – I know not where it came from - was stuck part-way down, which eventually fell to the bottom.  However, we still could not get it out.  
The eventual solution, (similar to Zander's), was to put a small block of wood on the ground, then lower the boat so that the keel was pushed a little further up and the offending piece of metal could fall out.
Greasing - I do – but if you look back at previous posts, I think the jury is out!
Good  luck