Sorry to bust in on your thread Ukchris, but I'm learning from this also!
Can someone clarify for me, is it possible to keep a group of angels 4-6 happily? Is it one of those things where you may or may not have problems?
It's not impossible, but you'll need a big tank and some patience finding the right mix. They basically will get territorial and chase off others they see as a threat to them. I had about 4-5 a long, long time ago but you're better off starting with a pair to see how you go. Even then you might need to get 4-6 as Techen says, wait until a pair forms and then return the others.
If you have the time, effort and patience to spare then you may be ok.