back Bee
I have my back garden cat proofed with Secur-a-cat and would highly recommend it. It's been up for over three years now with no escapes and Riley is amazingly athletic! It's really not that obtrusive at all, I was worried what the neighbours might think but went and asked before I did it and they were all fine with it - I did slip into the conversation I didn't want them soiling in their gardens
The majority of systems attach to the fence, when you say the fencing is not great do you mean you don't think it would stand up to netting/brackets being attached? The only free standing one I know of is Purrfect fencing but if I remember rightly that was very expensive, it might even be cheaper to get a new fence put up! Trees can be dealt with by way of a collar round the trunk to stop your girls climbing up it and using it as a launchpad but obviously you have seen the garden so can guage if the plants are going to be an issue.
This is a really good link as it's a website made by a bloke who installed his own cat proofing and gives all the options available
http://www.mycatfence.co.uk/choosing.phpIf you felt up to DIY then here's a step by step guide I scanned from an article in a magazine a few years ago on how to do a DIY cat fence...
Have scanned the relevant bits instead....
Will try and dig out a few pics of my set up...