Spot on Andy. I took it apart and did as you said. I had to click the mechanism rather than the lever but after a few clicks its working. Use it or lose it!
Submersing a sticky shifter in a strong mix of degreaser with very hot water has saved a few shifters for me. Good squirt of a water repeller after and some white gr see into the shifter mechanism has gone some really stuck Ines working like new