Even in service, I don't think there's any such thing as an SLA on ADSL. If your business depends on internet connectivity, you should have a leased line of some sort.

Or at least an LLU DSL service (such as with Easynet), and let them deal with and pressure BT on your behalf. After far too much fannying about at one of our venues between an ISP and BT, I went down this road. Much easier to deal with (and the line cost is part of the service, no account with BT necessary).