Cheers everyone! I have convinced my boyfriend (who just so happens to be a builder ;-) ) to build the enclosure this weekend. I think what we are going to do is make the existing fences cat proof because the garden isn't big enough to just enclose one part of it. There are a line of trees at the back of the garden just infront of a fence so unfortunately we are going to have to build a whole fence in front of them to stop them climbing them and getting out. This is a bit of a shame as Phoebe loved hiding in these trees then ambushing insects! Vinnie has never been outside as he is still small so it won't be so bad for him.
We were thinking of building a climbing frame structure to put in the middle of the garden too, so they have something to climb.
We are not going to have the cat flap accessible to them at all times though - just incase they got out of the garden somehow while we were at work :-S I think we will get a lockable one so that when we are in we can have it so they can go in and out. I don't know I think I just worry too much!
Obviously I will post pictures of our efforts, although it will probably take a while!