When we arranged for a digital upgrade, we also ordered another STB to be installed bringing our total to three. I remember reading in the Terms & Conditions that an account holder (ie One house) would be permitted to have a maximum of 3 STBs installed.
Surely if kzthou has five - service capability may be compromised. I am in know doubt that NTL knew there would be 5 boxes installed and perhaps should have declined install. Then again, this is NTL we're talking about and the people at the top want to make as much money as possible - regardless of the quality of service to the customer.
I think that if I was paying £150 per month for the five boxes and not receiving an acceptable level of service, I'd either uninstall the whole setup and go elsewhere or reduce the number of STBs to three and see if that worked - you'd be saving a few pounds as well, which is a good thing when it comes to NTL.