Thanks for the advice Des. But I'm having a little trouble visualising what you've done with your set up. You couldn't post or email a picture could you?
I've bought another tank, 30" (76.2cm). I'm currently reorganising things in the room to get it in it's final place and start setting it up. I've got a few more questions.
Would a dark substrate be better than a light one? I could maybe get something like Aquagrit, which is an inert substrate, quite dark and rather coarser than sand if I remember correctly. Would that be preferrable to, say, ordinary coloured, coarse sand?
Also, what about a few tankmates? I was thinking more specifically about things in the tank that might be useful, say, for cleaning. Like Otos or something. Will probably put a few lemon tetras in at first, to help mature the tank (I will also seed the filters from the other tanks, etc). I'll get as much water as I can from the place where the fish are so it's not a shock to their system. They are currently in ordinary conditioned tapwater. So I will have to set up that way and gradually lower it. Water around here is normally around pH 7.8 and quite hard. I have an RO unit, so my tanks sit around 7.2 at the moment. I've only had the RO unit since around August/September and am slowly lowering the tanks with ordinary water changes.
Incidentally, I have already secured two fish. They had a bunch in (well, around 6 or 7) at one of my favourite shops and I've been wistfully watching them for months. Slowly they've been sold off and I didn't know what to do because I really can't afford them. But I've been keeping an eye on their predicament and decided it was the time to do something before it was too late. So I went back and there were two Zebras left, so I put a deposit down and they are happy to hold them while I finish paying for them. £95 each. Ouch. I haven't told my partner yet!

I realise I will probably need more to have a proper breeding colony, and will be keeping an eye out on this forum for people in the UK who are selling stock. Yikes... I know I'm doing a good thing, helping save this wonderful fish. But I feel so guilty! So much money. But I can't imagine NOT doing it... I'm really quite excited about it!