Thanks wolfpack, I have now confirmation from the previous owner that he bought the battery several years ago without being precise as to how many years. He also confirmed that he had problems keeping the charge and had bought a special charger to deal with the problem, which he offered to pass on to me when we next meet. Meanwhile I keep on top of the problem with a normal car charger, knowing that sooner or later I will have to buy a new battery.
thanks again, gipsy wagon

 

If the lighter socket does not have power when the key is out of the ignition, then a charger plugged into the lighter socket is very unlikely to charge the battery when the key is out. Your solar charger is probably only powering the switched accessory circuits like the radio.

 

very possible but then it is also preventing those circuits from draining the battery possibly at least am hoping so will try running it directly to the battery and see if the radio still comes on when the key is out .

 

The switch is preventing those circuits from draining the battery.

 

Just had my XK8 MOT'd and serviced by a special XK8 garage. I asked about my previously mentioned battery problems and he confirmed that the small lid inside the ignition lock should close up in order to activate sleep mode. Mine didn't until I sprayed it lightly several days with WD40. The battery holds its charge now much better. Incidentally he checked the battery and found it in good condition. I presume the combination of charging every few days for a while and making sure the ignition lock closed properly AND locking the car (apparently necessary to activate sleep mode) all helped to put it back in a reasonable condition.