Neil,
I have been told by D-Links router tech support that there is an incompatibility problem with some of ntl's STBs and the DI604 router.
I was getting all sorts of odd issues, connection kept dropping, slow speeds, etc.
You need to contact the D-Link tech support, and make sure you speak to someone who specifically deals with cable routers - see their website
www.d-link.co.uk
They offered me a free upgrade, although is did take a little while to come through.
What the incompatibility is, I dont know, but the DI804 I now how seems to work OK.