Although everyone is perfectly correct in their advice, I do wonder if this is what the OP wanted to hear.....lol Maybe he/she didn't want to get rid of their fish? You're in the UK, so I'd suggest that assumming you are allowed to ( cos not everyone can just go out and buy 6 tanks, I wish I could), go to Pets At Home and get a few of those 'My First Aquarium' things they do. Basically they are a 3-ish gallon tank. It's a no frills job, they cost £15 and even come with a small filter, (plus some tacky fishy stickers, those are optional to use....). I have one but I find the filter far too powerful for the tank size, so I use it intermittently until I figure out how to dampen it's output. But all in all, they are quite good little glass tanks that will fit anywhere. A heater can be bought on eBay, I use a small 6 inch 25 watt and there is no problem. Of course the tank(s) will need frequent water changes until you can get something either better or what ever else you decide to do.
Another alternative is a clearseal 4 ft tank and divide it? They are quite cheap and you should get away with dividing it in to 4 without any problems, then either divide the original tank or get a small seperate one for the 6th fish.
I hope this helps, but in case you get stuck, I'm sure I have room for a couple more bettas.....