Trackpad was sticking.
All went well apart from two stripped screws which I had to drill out. Repair guide was very straightforward. After I'd reassembled the machine I found that the battery had expanded which was preventing the trackpad from moving - so replacement was almost certainly unnecessary. Useful experience though!
On the older MacBook Pro with external battery check that the battery has not distorted as this will press on the underside of the trackpad preventing it from working.