tarmac does good prices on the invidia,
but like a said, a custom build is cheaper and comes with a life time gaurentee, buitl to your spec and sound preference, and all should be less than £500
Here are some pics of my build, mine is a full true dual exhaust but with an X pipe in it and 4 silencers. they also built me my own racing cats as well, but that all bumps the price up.
If you went for a true dual design with say an H pipe connector 2 silencers should easily come in under £500