Thanks for the advice, everyone -
@Grumpy_Mike - I've ordered a load of Schottky diodes to play with. Managed to find some on Farnell that have a forward drop of around 0.25 volts which sounds nice and efficient. The resistor-across-the-diode(s) trick sounds good - it'd never have occurred to me. Seems so logical, thanks!
@Mauried - points noted. Even if the batteries started off with the same open-circuit voltage each, when you take a reasonable load from them their voltage drops / sags, and then once the load is removed they take a little while for their voltage to bounce back. I'm guessing that batteries in different conditions (age / capacity) will sag and bounce back at different rates, meaning they'll recharge at different rates too.
The main thing is that even if they don't charge equally, I should still get an increase in usable capacity even if it isn't as much as you'd get with a more sophisticated per-battery monitoring and charging system. With SLA batteries, it seems the key thing is to spread your load demands over as many batteries as possible, so if adding another old one - even if it hasn't much capacity - reduces the amount the others will discharge by even a small amount, it'll be helping.
I know that I'd get a more efficient system by running things at a higher voltage, but the convenience of having the whole setup (panels, batteries, lights etc) running at 12 volts also translates into savings - no need to keep converting 24 or 48 volts down into something usable.
@AnthonyMartello - a generator would be a great backup to have but I'm trying to build a system that doesn't rely on fuel supplies. I figure that if we do have an energy crisis the solar panels will keep us lit and entertained and we'll hook an alternator up to an old push-bike as a way to supplement it if needed. I like the idea of a bike-powered TV 
My aim isn't to have a perfect system, just a cheap and safe one, and one I can replicate and explain easily for friends and neighbours. If I can make use of old spare car batteries just by adding a couple of diodes to them and connecting them up to the bank then it'll cost me next to nothing and it'll save the batteries from being thrown out. And so far, I've spent less than £800 on solar panels, charger and fuses, and the system's given us free lighting, music systems in most rooms, iphone and iPad chargers everywhere. It's a bit hacky but it's worked perfectly for over a year now. Bring on the econopocalypse ...
I'll report back once I've done some experiments. Everything hinges on whether we can fool the charger into thinking everything is working normally.
Again, many thanks folks, your time and expertise is hugely appreciated.