Of course the diverter is only any fun if the taps are on. If the taps are off then it will be dry It may have got wedged with the little blobs of solder and crud that plumbers leave inside the pipe amd which come out of the taps over time.. I would open the bidding by trying to pull and push the knob up and down and if it starts moving turn on the cold tap to flush crud and keep wiggling it up and down. Then dismantle it , and clean it.
Thanks for your input, folks! I eventually sorted the problem by unscrewing the knob a little, inserting a bolster chisel underneath and gently levering up. Can't have been too badly stuck, just a little stiff as it came up quite easily. Had to admit defeat on another task - changing a basin tap washer as the whole thing was totally tight and I forgot to bring my big wrench. Been a fun weekend as my grandson broke his wrist when he fell over at a skatepark and my daughter's bf walked into a low doorframe and cut his forehead open.