Changing the cable seems to have sorted it,so I'm assuming that perhaps there was a shitty connection on the other one.
I'd phoned them and they tested the line.
Yeah, I've heard that one before. But they only test the line there and then and can't account for other factors.
I only say this from personal experience having both dealt with, and worked (for a short period), within the industry.
I've had a similar issue recently and thought my experiences may have helped, but if you've sorted it, cool beans.