Thanks everyone - it is a scam- they started talking about a 'delivery charge' to bring the cheque round, so all contact from our side ceased at that point.
I always ask myself ... (e.g.) This is very good of them. Why are they letting me know? How did they get to know whereas I, the ticket-holder, don't know? What's in it for them - why should they bother?
I am very well aware of the 'If it sounds to good to be true ...' motto, so my daughter and partner entered communication expecting the scam element to surface at any moment, and they were not disappointed.