try a good hardware shop, your only looking for a O ring seal, you should be able to get one that will do the job if you have a idea of its size, from car shops/ maplins/ ebay/ i have seen boxs of them in different sizes here in dublin, im sure you have a lot better hardware shops in the uk, dont get stuck on getting Aldi one, try b&Q ,, ect..
k, February 2012