Update, engineer came round today and despite the awful rain did a good job, he checked and found the cable is indeed damaged (shorting to ground somewhere between me and the "pit" along the street), and someone will be out to replace it in a week. He also replaced the fittings at the box while here as they had seen better days and one cable was heavily kinked at the connection. Credit to him, very professional, understood the issue straight away and found the fault. Shame it takes so much pain and effort to get to this point, but I can't fault the engineer in any way.
I'll (hopefully) post on this thread only once more in just over a week when the cable is replaced and fingers crossed all will be good.
Yup all good, no drops at all today and after a few hours there have been no post rs errors at all, and the pre rs are in the normal and expected tens rather than the tens of thousands! Again getting things fixed was a way too drawn out and painful process but it can be done, hope this thread and results help others.
Downstream bonded channels
Channel Locked Status RxMER (dB) Pre RS Errors Post RS Errors