In the end, I chose Axa for Disney. £7000 per period of insurance for £17.49 a month (7 years old).
There wasn't any introductory offer (I was going to choose Sainsburys which had 25% off a new policy but that quote was £26 so I dread to think what his premiums would have been next year!) or cashback deals but it was the best cover I found at an affordable price that, fingers crossed, shouldn't jump up too much in the future.