Advice needed-being pursued for gas bill that's not mine. Help!

Hello there,
I hope someone can give me some advice. This is my first posting (have been coming to the site for advice for years tho) so please gentle with me!

I am currently being pursued by a company called Buchanan, Clark and Wells (BCW) for a gas bill at a property I lived in 2 years ago but for a billing period after I had left.

The story so far- I left the shared rented house in Feb 2006 and moved into my current rented flat. I advised British Gas I had moved in here and the bill here is in my name. The bill at the previous property was in several names, but mine was the first on the list.
In Oct 06 I got a demand from Wescot Credit Services for £178.82 for a bill dating 7 Feb-17 Mar after I had moved out. After lots of to-ing and fro-ing I finally got a copy of the bill. When the dates were clear, I sent them a copy of my first bill at my current property and a letter explaining I could not be in two places at once and that I was not responsible for that bill. They agreed and closed the account sending it back to British Gas. I thought that was the end of it.
Then in Dec 07 I got 2 nasty red 'formal demand' and 'final notice' letters from BCW demanding the money. I wrote to them including the 2 copy bills and details of the correspondence with Wescott. They sent me another 'legal notification' and I then telephoned them and they said they needed proof of when I left the first address such as a rental contract, luckily I had the 'notice to quit' order for when we left so I emailed (didn't work) then posted this to them. This was all done in early January.
Two months later this week I got a letter from them saying they had a response from their client and I should call them. I spoke to them yesterday and they advised me that they now needed the final meter readings from the last property. I don't have these as it was over 2 years ago, and another tenant was paying the bill via direct debit. I do however know we were in credit when we left, and the bill is clearly dated AFTER we left! I pointed this out to the operator and said I didn't understand how I could be pursued for a bill that wasn't for dates when I was living there and he said he 'agreed with me morally but not legally'. Fine. He then told me I have til early next week to find evidence or it will go to solicitors and I will be forced to pay this.
I really don't know what to do! It is highly unlikely we will be able to find meter readings and I really don't what other evidence I could poss come up with. It seems like every time I provide something they want something more.

I am normally quite good at dealing with issues like this-I am paying off all my debts and dealing with redundancy at the moment- but my father died last wk so in the midst of all this I am sorting out practical arrangements and trying to organise all my mother's finances and bills to ease her burden and this pursual really feels like the last straw and too much for me to cope with right now.
I am going to contact the CAB on monday but any advice that anyone can give would me so so welcome.
Sorry for rambling on and thanks so much for your time.