They are sometimes listed om E-bay, but not so easy to find.
Till now I saw about 10 sets passing by wich I posted on some forums.
At the moment nothing listed, but I keep a eye on it.
I tracked down a parts dealer for Land/Range Rovers in Denmark. They have a replacement rubber grommet which might fit.
I sent them the pictures of the Range Rover part that you (Oetker) bought and they say it is not identical, but should fit as it has the same size.
I have ordered a pair and should have them early this week.
The company can be found at:
www.llrparts.dk the part is here:
http://www.lrparts.dk/shop/search-1.html?keyword=gennemf%F8ringIt onlt cost DKK 76,56 incl VAT. ~ 10€ or 8.25£
PS. curious detail about the original grommets.
I have an extra door where the original grommet is still on the wiring harness. It is also broke, but an interesting detail is, that it is actually twice as long as the visible part. There is a smooth extension that fits inside the frame of the car. I have not had the inside panels off to see what the purpose of that is supposed to be.