Handy tip: Regardless of how waterproof a product claims to be, it's always a good idea to put your things inside a plastic bag inside the pannier.Thanks to all who replied. I popped into Decathlon yesterday and picked up one of their messenger bags that clips to a rack. It's smaller than a conventional pannier, but I wont be taking that much stuff to the office. Packed lunch and a couple of tubes, pump and lock etc and still have room for the milk or whatever I'm told to bring home.
It looks sturdy with decent fittings. We'll see how we get on.