Solved it! Sorry if it's obvious to everyone else, but it wasn't to me until this morning!
There are 2 levers on the side of the seat, labelled A and B in the first picture. Lifting "A" does two things: it allows the seat back to fold a bit forward, and releases the seat on the rails. I presume this is to allow people to get to the rear seats if fitted. Notice that the lever stays up so that in that position the seat is free to slide. If you then lift "B" you can fold the back right down, but as "A" is up the seat still slides. This is what I was doing.
But if you don't lift "A" at all, and just use "B" you can fold the back right down but "A" is down and the seat remains locked on the rails. Hooray! As shown in the second picture.
As we normally have the seats facing backwards the levers are in the middle of the vehicle which is why it was difficult to see what they did.