Thanks Mick. I tried that but couldn't get it to budge, and since i wasn't positive that was the route to go, I quit before I mucked it up! Just about to take a short hop over to Nanaimo for a week.
I suspect that the diverter itself may be OK but there might be some debris on the cold side that was blocking it, and as Bill mentioned, is now moved out of the way (though also temporarily).
I'll order in a new diverter anyway, and work on dismantling the cold faucet to check for stray bits 'n bobs... but I will wait until after we get back. If I have a new diverter, if I muck up the old one in the process, it won't matter much.