Manfrotto replacement centre column needed

You could also try a strap wrench:

http://www.24retail.co.uk/index.php?main_page=product_info&products_id=508480

Vic.

I'd simply screw a bolt into one of the three holes part way not far enough to lock the plate in but it would give a place to stop a wrench from slipping, and then take a plumbing pipe wrench to it, on the aluminium part that is already broken...

There is another option ..... buy one of these and make a new part:

http://www.chesteruk.net/store/conquest_lathe.htm

Vic.

Sorted.
Thank you to everyone for help and suggestions [even Xbow's suggestion to buy a lathe !! Ha Ha]. I ended up drilling a hole on the side of the old head after all - and one knock with a punch loosened the head which then easily unscrewed. Then - the protruding screw head needed to be replaced - then I did glue the broken part [fingers crossed on that]. And eventually is all back in working order.