You're right, thanks. I was on the American Amazon, I think, where they were writing about it being withdrawn but I've just found the set on the Naxos web site so it must still be available. Also amazon.co.uk say they have 4 in stock.
In fact the Naxos web site says "Not available in the United States due to possible copyright restrictions" - so that explains everything.
I've had a really enjoyable evening listening to Act 1 of my recording, what a wonderful work Don G is, definitely my favourite opera. Funnily enough I'm not too keen on Mozart in general!