I would agree with the secondary battery solution. rig two good sized ones in parallel on the floor/shelf and then load them across car battery in parallel. You can then rotate one of the two to your home to load it fully from a conventional charger in turn. This does not power cycle or manage the battery but just loads all three to give a deeper reservoir
Alternative is a more complex solution ?
Use a trickle charger like the CTEK MXS5 to manage the battery.
http://www.ctek.com/gb/en/chargers/MXS%205.0 Your issue here is giving it a 240v power supply and charging this.
You need to collect with power with solar/wind (CTEK is 6A@12v=72W, so aim to double this for loss) so you would need something like a 120W panel at a guess.
http://www.maplin.co.uk/p/120w-solar-power-station-n46hathis is an example kit that gives a panel, power inverter, battery to even out and setup guide. £900 all up. In theory it could also charge some other small items (keep air flowing for damp issues or an alarm) that might assist you ?
Feels like pitching a battery back and forth might be most effective. Don't forget they are full of acid so if you do then get something that is designed to protect everything in case of it tipping or being hit. Worst case is a car accident and you being hit but 20kgs of flying acid in the car. Also get a mechanic to walk you through the correct connection order and safety. (sorry if this is grandma and eggs, just trying to avoid a melt-down)