It's the contention on your ISP Rob. If you want faster, you need to ask if they can upgrade you from 50:1 to 20:1, although it will cost more. You get what you pay for.
You could switch to Talk Talk, although the chances of them having done your exchange is 0:0 (they do allow 30GB's download a month, unlike BT, Tiscali, who cheap tariffs allow 5GB's at most on Fair Usage).