To coin a phrase... Eureka!!!!

I've just realised my mistake- I've put the MAX4544 in the wrong way round.

I have absolutely no excuse, except that it was quite close to the end of the instructions, and I was obviously getting careless.

Flipped it round and it works perfectly, even with no aerial plugged in.

Thanks for all the suggestions - and to Patrick for a great kit, and the patience to listen to us lot whinging about our inability to follow simple instructions :-)