*If* I'm right, then you need to find an ISP who have their own kit in the exchange. Otherwise your fault will just get migrated to your new service, as it retains it's BTw equipment. I guess If LLU isn't an option on your exchange, then I'd suggest A&A, or Zen, who are 'aggressive' in chasing faults on their customers behalf. Both UK based call centres also.
Good luck, and keep us posted.