I bit the bullet and changed the cell and all is functioning again. I also got the salt down to 3800ppm (according to new unit) so I'm letting it go at that.

I had the local Hayward guy come do the work, partly to get it done as quickly as possible (going out of town, of course - my limited experience suggests that pools always stop working right before you are about to leave them for a few days...) and partly to have experience on hand if it turned out NOT to be the cell. He said that AquaRite has changed its panel configuration in the last couple of years to address the not-infrequent premature burn-out that has plagued the earlier models, so the newer panels are much more likely to live for longer. (I asked about this because I have had to replace a burnt out capacitor on my panel already, and it was only two when it burnt out! Capacitors croaking was the issue with the premature burn-out, which Hayward has now fixed.)

And I have ordered the salt-checking kit!

Thanks again for all the help and wisdom.

Alison