I have discussed this with BT / OFTEL (OFCOM now I think)
Firstly, OFCOM is a brand new organisation with a brand new management structure, so lack of success with OFTEL does not necessarily mean that OFCOM will not help.
Secondly, I can understand BT not taking action for the reasons you give, but why would that deter OFTEL?
Have another go at OFCOM and, if you are not satisfied, PM me, as I sort of have an inside track there.
Timothy