I thought id quickly update this!
It turns out it wasn't the rear light seals!! Although they were knackered it turns out the water was getting in through the passenger side, side trim which has GTI on it.
That little bit of trim is held on by four clips that go into holes and rubber bungs inbetween. the clips were slightly broken and not pushing the rubber so water was able to seep through!
worth noting if you have a mystery leak!!