Indeed, Talktalk have always run their own LLU ADSL DLM system. It's much better than the BTw system in that you can turn the bloody thing off.
Even if they wanted to, Talktalk can't just make their own DLM for FTTC. OpenReach own and run the FTTC network very differently. There's just not the flexibility for an ISP to customise GEA-FTTC. It's OpenReach's DSLAM (Fibre cabinet) unlike LLU ADSL where the ISP owns their own equipment in the exchange.