So I have to pay £75 to do this, and even then there's no guarantee that it will happen?

Hi Guys, Bit of a problem, and I'm not sure what to do about it. When I was at university, our household subscribed to a monthly electricity plan from a company called "Glide" - we were explicitly told that it was an unlimited plan (within their fair usage), and we all paid a set amount every month (it wasn't cheap, but we thought it would be better than worrying about whether the heating was on, etc). We all moved out after a year with no problems - fast forward to January, I get a phone call from Glide saying I owe £110. I express surprise, and ask why. Apparently, the monthly deal wasn't "unlimited", and was actually just an "average allowance" and over the course of the year, we'd gone over this limit to the amount of £110 per person. I argued back and forth that I didn't owe this, because we were explicitly told that it was unlimited, and we received absolutely no correspondence from them that we were going over their allowance. The conversation ended with the woman on the phone asking for my email so she could forward through a copy of the invoice, and it would hopefully make more sense. I received the invoice (dated September 2009) and that was that. I had no further communication from Glide - no emails or phone calls. Today, I received a Notice of Application for Attachment of Earnings Order. Apparently, a Judgement has already been made against me, and I now owe £318. This is the original debt plus interest (£218) plus a £100 Attachment issue fee. I've literally no idea how to proceed with this. Glide had made a single, flimsy attempt to get me to pay back in January, and even then, the woman seemed pretty eager to end the phonecall and just send me an email. I never received any "judgement" from the court, and this is the first I've heard of any of this. I'm assuming it's now too late to pay Glide directly, but I object to paying almost 3 times the amount of the actual debt (which is bogus anyway, in my opinion). Does anyone have some advice, please?