Realise or Realize?

[megapixels]

Seems obvious which one is correct, but DD (Year 4) was doing a worksheet which had some "dictionary work". She had to look up 'realise' and there was no such word in the Oxford English Dictionary for Schools! It listed 'realize', but I thought that was the American version?

I am old and was taught that 'ize' was correct at school and 'ise' a variant.

I knew about the Oxford thing because I remember watching an episode of Morse where he realised who the baddie was because she spelt something ize rather than ise. She had been at Oxford with Morse and her spelling gave the game away!

- love that MrsW!