So the BT engineer arrived today, he was the same engineer who attended prior to Christmas. He phoned me to tell me he wouldn't need access to the property as he'd be working at the distribution point. He was going to switch my line over to a completely new circuit on the cabinet to see if that sorted the problem. He got half way through before it started getting dark, he's going to return first thing and finish up. In the meantime he said I'd still have an active connection without any issues. Touching wood since he's been no disconnections yet, will monitor from tomorrow....and update.