...and the winner is! The rear exhaust silencers are knocking the rear subframe/towing eye.
So my brother has been down, and I've just had a crawl underneath the car. Even though it sounds like it's coming from under the drivers seat, it's actually the rear pipes knocking the subframe slightly, and that sound is resonating up the whole length of the pipe so it's really hard to pin it's location down.
I have replacement CATs from Cats2U and they don't fit very well. The joint from the header pipes to the centre silencer is lower than it should be, and this in turn makes the exit higher than it should be, so they're knocking ever so slightly when you put the car in gear, and the suspension dips before you pull away.
If I could bend the header pipes slightly I could probably make it line up better, but I haven't got the tools for that so it'll have to stay as it is for now. Good to know it's not the engine or gearbox about to fall off though! That is a relief!