Current situation:
Connection getting worse, getting less than 0.1 Mbps when connected which ain't often. Received modem/router from Orange yesterday, installed it and got exactly the same problems. Speaking to Orange during the install agreed that there must be a line fault but that I had to ring back on a mobile because thay couldn't register a line fault report while I was on the line.

I asked if that meant the line would be checked immediately. No it takes 48 ~ 72 hours

So why can't I report the line fault on the BT line rather than spend a fortune calling on a mobile? Anyway I call on my mobile, only problem is that I often don't get a signal in the house so I speak to them from the garden. I have to go in to check the status and the line drops

Finally get them to report a line fault.
This morning I call BT and report a line fault, there's a definite crackle on the line. BT won't send out an engineer unless I can confirm my phone's not at fault. I don't have spare so, keeping BT on the line, I go next door to see if I can borrow a phone. Get accosted by their dog but decide it's too much effort to get their phone disconnected so I leave it.
Have to do the school run and then a hospital visit so BT said they would ring back this afternoon to see if I've proved if the phone's at fault. I buy a phone at Argos for £4.95(!) and now I've proved that the line still crackles.
Rebooted modem/router and been working for some time now <fingers crossed> and will speak to BT later.
In the meantime another modem/router has turned up!