Thank you Stuart - I'll have a look although I have been told that "Event Insurance" won't cover this sort of thing. After trying every insurer that I could find I was eventually told that no policy existed to cover this need and, if I wanted such a policy, then one would have to be written especially for me and then the premiums would be so high I wouldn't want to pay.
I have asked other artists who organise painting holidays and they seem prepared to take the risk.
I have got round the problem by doing painting holidays with another artist. For instance Joe Dowden and I ran one in Croatia between us which worked quite well (especially as Joe broke his arm really badly on the second day and had to spend hours and hours in a Croatian hospital!) However this sort of joint venture isn't always possible, so now I pay a small annual retainer to another artist friend on the understanding that, if necessary, he will take over as tutor. So far he hasn't been needed but it's comforting to know I have back-up.
However, it isn't feasible to expect someone else to take over at a moment's notice if the holiday is abroad, so now I no longer risk the foreign ones but keep to painting holidays in Britain.