Ok we need a pretty high output mic for our singer. We've got big enough amps/speakers to run her through, but the mic we're using is from a karaoke machine and, subsequently, is too quiet.

So can anyone recommend a good mic thats not too expensive. We're willing to go to £80 but cheaper would be better.

I think it should be around that price range.... I *love* my Sennheiser e835 for live vocals. It is a big step up from the usual SM57/58 breed.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
I'm seeing alot of reviews saying that the berhinger xm8500 mic is pretty good and that it is very comparable to the shure sm58. The XM8500 is pretty inexpensive as well. I recommend searching for it.
Shure SM58 is pretty much the industry standard for a live vocal mike. Somewhere between £60 and £80, depending on where you get them from.