Fifty years ago, the rule for British English was that if the word derived
from ancient Greek and had a zeta in the suffix it should use '-ize',
otherwise '-ise'. Words with a sigma in the suffix were supposed to use
'-ise' even then. Anything of Latin originthis was always supposed to
use'-ise'.

Nowadays common usage is -ise for almost everything. For a fuller
discussion, see page 28 of The Times for Tuesday January 13 2004.

Regarding Rik's spelling checker, I find that with Microsoft Word set to
British English, I'm still sometimes reprimanded for using '-ise'.

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John Rabson G3PAI