To summarize other discussions that I read when I was in a similar situation:
1) dry-erase markers will eventually stain the board
2) wet-erase markers are used by some people
3) dry-erase crayons are a good option (Crayola brand available at Walmart in the USA).
I went with dry-erase crayons. They are cheap, easy to find and give good results.
Thanks both for your input. I went in search and found the thread about dry-erase crayons and ordered some earlier. Not quite as cheap as in the US but not bad. I look forward to playing with them, as long as they arrive one day before the final estimated delivery date of 15th January. I've booked in a play on 14th!