OK in that case I would simply sit a weatherproof solar charger next to the car (or even on the bootlid), connected directly to the battery in the same way that my AccuMate mains charger is. I've got a short flylead permanently attached to the battery terminals, ending in a 2-pin socket (with rubber cover) that is tucked away in a corner of the boot. The charger lead has a matching 2-pin plug. The bootlid obviously doesn't quite shut with the lead going under it, so make sure you flip the switch to turn the boot light off (or the bulb will pull more power from the battery than the charger is putting back in!). This switch resets when you close the boot, so you have to remember to do this each time you put it on charge.
If you don't want to mess about making leads yourself you can buy the AccuMate ones separately:
http://www.accumate.co.uk/cg020001.htm
They are pretty cheap e.g. the fused one to go onto the battery is only £3.53 (or £4.70 for the one with the rubber cover). I had to open the eyelets out slightly to fit the bolts on the battery clamps on my SL (they are obviously bigger diameter than normal!).