Actually Daz that's really not quite right. Ofcom (the telecomms regulator to which BT is bound) forced BT to completely separate the local line part of the company into a new (subsiduary) company called Openreach.
All telephony, and broadband, suppliers have to use Openreach for the local link for consumers (like you and me) connections into the service providers service (which is at the local BT exchange, although accomodation and space there is provided for all competitors to make sure that BT don't have an unfair advantage by virtue of having all the exchange buildings around the country).
Openreach have to ensure, demonstrate, prove, and report-back to Ofcom, that they offer all ISPs and telephony companies with the exact same service (...called "equivalence"), and that they don't for example favour BT.
BT I believe has only about half the telephony market nowadays. But you're right in that all companies have to use Openreach for the local line, so there's a monoploy in that respect.
But a cartel is where a number of companies gang up to artificially fix a high price - and that isn't what's happening here. Though I think we could talk about petrol companies eh!
Anyway, good luck with the claim.
p.s I ain't talking for, or representing, BT here by the way