I sympathise. I had been with the same company for years, thru 3 name changes till it became ntl. No problems. I moved the same day they became Virgin. OMG did my troubles start. Even with the same staff they acted like they were virgins on the telecom scene. Broadband madam? Yes please. I was set top. In the new place they fitted a modem. Just ring up and get a number to install. I did. It didn't work. They said, 'ooohh, that number would never have worked'. I said 'get me connected tonight or you can come back and get your stuff and I am going to Sky. He then said he couldn't connect me without going on DD. 'What?. You want me to DD for �8 a month but no requirement for the TV which is over �100 a month'. Yes. I did. They didn't manage to call on it. A red letter came. I paid my entire broadband bill for 7 months, the duration of my house rental. It took the woman 3 goes to add it up.
The equipment was brand new. 2 months in the broadband slowed to a snail. Techy came in, set the modem to higher setting and said if that didn't work I needed a rewire. A what? Your wirework doesn't last longer than 2 months? Not one piece of equipment, modem, 2 boxes, remotes, all new from boxes, worked correctly. I even took a new TV back to the shop to see what was faulty. It wasn't the TV.
Now with Sky. No problems. Touch wood. Tho I am sure many would tell you a similar story of defecting from sky to virgin. Luck of the draw.
As you say, there is no written record of a lesser charge so it looks like they have you. Well done for getting a 6 month rate for 18 months but you may have to bite the bullet now. Whether its DD or other payment method, they are intent on getting it.