I may have mentioned this before but last time, my actions and the timings may have been a bit woolley. This time, I have it down pat.
My car is a 1995 XJ6 3.2 with no modifications that I know of except that it has an aftermarket Panasonic RD525LEN Radio Cassette with a 6 CD changer in the trunk/boot.

I have 2 car keys and 2 alarm fobs.
Roughly 2 weeks ago, after suffering a similar problem, I put 2 new CR2016 batteries in each alarm fob and did the settings thing with the Valet switch and open boot/trunk and driver's door. Both fobs worked perfectly, even at long distance and from inside the house. That was right up until I locked the car last night.

This morning, zilch, nada, rien, nothing. Not a peep out of the main fob that I use all the time. I got the other fob out and tried that and it was just the same. Nothing.

Now, forewarned by what happened last time, I didn't change the batteries. I just did the Valet switch thing again this afternoon after trying the fobs again with no success.
After doing the programming again, the fobs both work perfectly.
What the smeg is going on?
Cheers,
Steve