Right Twevy, here's the score. What you might find is that if the work to be done is still under warranty then the dealers will be only too bloody pleased to get your vehicle in to work on it, 'cos THEY aren't paying and YOU aren't paying. It's a terrific earner; but it's a right pain in the Uranus for you, driving backwards and forwards to the dealers. This happened to me, two years ago and it drove me nuts. Drop me an e-mail if you want to know the dealers and I'll let you know, 'cos I live in your neck of the woods.
I take it you've got a sun-roof, 'cos that's the root cause of the whole problem. Yes?
Well, this is what causes it, here's how to locate it and this is what to take with you next time you go to see them.
Firstly, there are four drainage points from each corner of the opened sun-roof. For each of those drainage points there is a corresponding rubber hose. If either the drain points or the hose is blocked, rain-water backs up, builds up on top of the head-lining and floods either the front or the rear light-dome. Here's how to locate the problem (don't try to sort it yourself; the headlining is a batsard to remove, damages easily and costs a mortgage to replace) plus, of course, any yellow wires that you encounter while doing that have been re-cycled from a suicide bomber's vest, so's the explosives they're attached to.
First, let's check the rears. Lift up the tailgate. Looking from the back of the vehicle you'll see two rubber grommets about the size of a fifty-pence piece at about eye-level. Those are where the rear drainage tubes come out. Point the car a tiddly-tad uphill. Open up the sun-roof. get a jug of water and, using a step-ladder (yeah, yeah, I know, you shouldn't lean in sideways from up on a stepladder. There ain't no other way to do this. Get your mate to hold it) lean in over the roof and pour some water into the gully that runs around the edge of the sun-roof. Run round the back of the car and check whether water comes out of the drainage grommet on that side. DON'T get your mate to do this...aah sh**, too late, the ladder's on the floor and you're on you ar*e, right?
Do both sides. If no water comes out of one of them the hose is kinked (common) or there's some masticky stuff in the gulley (dead common).
Now check the fronts. Do the same with the water; car either level or pointing a teensy-tad DOWNhill. This time the water should go down a tube that comes down the A-pillar alongside the windscreen and dribble out underneath the car just behind the front wheel-arch.
Do all that BEFORE you go down the dealer's and they'll know 1.) what they SHOULD be looking for and 2.) they AIN'T dealing with some muppet.
Let me know how you get on.