UK Landlord Forum / Life

I was in a joint tenancy from August 2010 up until 5th August 2011 with 3 other tenants, jointly and severally liable.
One tenant did not pay his rent since February. The letting agency did nothing about this until the last month or so of the tenancy. Two different people also ended their tenancy there due to violence and threatening behaviour from this tenant, and so were replaced. The letting agency said this was not of their concern and to call the police.
I could not leave the tenancy as my deposit could not be refunded if the rent was in arrears from any tenant, so I could not afford to move. The tenancy also needs to be agreed by all tenants in the property, and the tenant informed me that if I wanted to move out, he would not sign the forms to end my contract. This problem had already occurred when a previous tenant moved out.
On June 18th, more violence and threatening behaviour and personal property being broken and stolen led to the police being called again. I began to pack my belongings the next day, and stayed at my boyfriends house at night from then on.
On 28th June, I returned my keys to the letting agency and informed them I would be moving out of the property due to the behaviour of the other tenant. I also warned the letting agency that they needed to do something else he was likely to damage the property, but they took no notice and said they would do nothing before the inventory check on the official tenancy ending date (5th August 2011).
I did not go back to the property after this date, and have been staying with my Mum ever since.
On 5th August 2011 I got a call from the letting agency saying the property has been trashed, e.g. maggots, faeces on walls, used condoms and tampons thrown around. The tenant still hadn't paid any rent, nor returned his keys. He sent the agency a letter claiming that he hasn't been at the house for 2 months and gave his keys to me as I was staying there with my boyfriend whilst subletting the other rooms.
Due to the joint liability nature of the tenancy, the agency are holding us jointly liable despite knowing that everyone has paid rent in full except for him, and the landlord is taking us/our guarantors to court to claim the damages.
I don't know if there is anything to do or if I have to pay hundreds of pounds that is not my fault. Surely the letting agency should have taken action back when the rent first wasn't paid, or when we kept phoning them saying we were scared of the violent tenant, or when I warned them he would damage the house. Isn't it their responsibility to take action? They could of prevented this.
I also don't know if it counts for anything if I could prove I wasn't there? My housing benefit and council tax were cancelled when I left and I've been registered for Jobseekers Allowance at my mum's address since. The neighbours could also hopefully confirm I have not been there, as could my job and my volunteering roles, all of which I had to leave in a hurry.
After having to quit my job, pay rent on a place I wasn't living, losing my deposit and trying to pay off bills that other tenants refused to contribute to, I really can't afford his rent and damages on top.
Thank you for any advice anyone can give me, I'm so worried.