The best advice I can give to anyone who has made several calls and complaints to the call centres, is email the CEO of the company.
A couple of years ago we were with virgin using a BT phone line, and got about 7Mbps, we then moved to the outskirts of the city, and stayed with Virgin, but initially only got a speed of 1.25Mbps.
This quickly dropped to an average of 0.18Mbps. I made several phone calls to the helpline, went through all their checks with them, and several times got to the stage where I needed to put the phone down try a few things and call back to let them know the results, but everytime I phoned back they insisted I went all through the process again and when it go to the put down the phone and try the test, they wouldn't take the results from the previous time, and refused to continue unless I did the test they said and phoned back. This ended with me having several arguments, talking to several 'supervisors',promises of notes being made on my account, promised call backs, engineers not turning up. It ended up with the internet completely dropping out for several hours, and then coming back for 30-40 mins, then down again.
I gave them fair chance, and finally decided enough was enough, I called the helpline again, gave them a final chance, took the name of the person I was talking to, and told them if they didn't stick to their promises this time, and didn't sort it or at least make some sort of noticeable improvement, I'd be contacting their CEO. This still didn't seem to bother them so I sent an email to the CEO as promised. Email sent to him at 8:30AM, containing account and contact details, by 8:45 I had an email from him, (actually him, not his staff) saying he was very sorry and his personal secretary would be in contact with me. By 9:15 I was talking to his secretary who put me through to a senior technician, I had a direct number to the CEO's office. Next day an engineer was out, replaced all the wiring and equipment in the flat, checked everything in the box outside, and the internet was ok for a few days. it deteriorated again. As they'd been out and couldn't find the problem, they said that I could cancel the contract and they'd waive the outstanding cost, AND refund all the bills back to the date they started. Never did find out what the problem was as we moved out not long after.
But the point being, if call centres are doing nothing, just go higher... I've got a list of CEO email addresses somewhere if anyones interested, includes car companies, supermarkets etc