Thanks for taking my call last night.
Due to the intermittent nature ofthe fault and the onset of nice weather, should the issue represent itself again please post back to let us know and we can take it from there.
Well I thought I'd update this thread with the details of the resolution.
Numerous Openreach engineers have attended but the mods booked a broadband special faults investigation engineer called Basil. Brilliant guy!
He changed the internal cable from the external termination point to the master socket as he didn't like how it had originally been routed. This seemed to help however the fault reoccured.
Basil was booked to come back and he ended up requesting another engineer to attend with him and they spent the majority of the day replacing some old junction boxes in two out of three chambers. (Not sure the exact name but apparently when the engineers come across them they are supposed to replace them). They couldn't access one underground chamber so the moved my parents line onto a different cable through that section and had to borrow a bucket and trowel to clear the bottom of another one as it was filled with a few inches on mud and concrete bits.
Touch wood the phone line is absolutely silent and the broadband speed is still slightly slower then they used to get it is stable and not disconnecting, but should it reoccur Openreach have at least made sure two thirds of the underground chambers are clear and free of old cabling and covered roughly a quarter of the distance back to the cabinet from house.