I've just finished a 12 month assured shorthold tenancy..
It ran out on the 19th February 2011.
They wrote me a letter asking if I was going to renew it and to sign below to which I did an sent it back .
In the last 2 weeks I have had a serious personal problem arise which means I need to vacate the property asap but I also received my new tenancy agreement today but its not signed an I haven't signed it either. Does this mean I'm tied to staying there or can I leave ?
Also if I can leave do I need to give them notice ?

This should be under real estate law so I am going to ask a moderator to move it so the right people can see it.

Both answers depend on where you live, what was in the form you signed and sent recently, and what was in the lease that ended. So you need to provide 3 pieces of information.
Of course you really should talk to the landlord ASAP; the sooner the better, regardless of the law.

Both answers depend on where you live, what was in the form you signed and sent recently, and what was in the lease that ended. ...

Yes.

And I hate to appear ignorant, but what is an "assured shorthold tenancy"?

AK its UK law. Assured Shorthold Tenancy Agreements - An introduction (http://www.tenancyagreementservice.co.uk/assured-shorthold-tenancies.htm) I am not up on UK tenancy laws so passing on this one.

...
Also if i can leave do i need to give them notice ?

Apparently so:

"If the agreement is for a fixed term, a tenant has the right to leave on the last day of the fixed term without giving any notice. If they stay even one day over the fixed term, they will automatically become a periodic tenant and will have to give proper notice unless the landlord agrees to them leaving."
Ending a tenancy agreement (http://www.tenancyagreementservice.co.uk/ending-a-tenancy-agreement.htm)

For further information, you might want to go to the site that ballengerb1 found for us. There appears to ba an advice forum there, but with stronger emphisis on British property law than we might have here.

Our tenancy was for 12 months which expired on the 19th February 2011...
We live in essex an all that was in the letter from them was how my assured shorthold tenancy was due to expire and did I want to renew my tenancy please sign an return which I did at the time , they've only just sent my tenancy papers through today and I've got to sign them but NOT date them oh and get a witness to witness it too

Any help is gratefully received

Kathy

our tenancy was for 12 months which expired on the 19th february 2011...
We live in essex an all that was in the letter from them was how my assured shorthold tenancy was due to expire and did i want to renew my tenancy please sign an return which i did at the time , theyve only just sent my tenancy papers through today and ive got to sign them but NOT date them oh and get a witness to witness it too

any help is greatfully received

kathy

It looks to me that the first letter was for the purpose of determining whether the LL should go to the trouble of preparing the formal agreement, and doesn't constitute a renewal agreement as such. If you want to move out, give them notice and then do so.