By lease I assume you mean tenancy agreement, and that you signed something when the first agreement was made which made you a guarantor. If the first agreement has subsequently been replaced by another I think that you will still be a guarantor if the wording of your guarantee document allows for this. You need to check the wording carefully, but I would guess the landlord (or agent) would have made sure you would be committed so long as they had that particular tenant.
Has the tenant left the property? I assume not as you refer to a possession order. The landlord should apply to Court for this and - if it is an assured shorthold - can use an accelerated procedure. Even with this it can be some while before possession is obtained, especially if the tenant does not leave voluntarily. Also, if the landlord knows he can get the rent from you he may be less inclined to hurry the process on.
I don't think you can apply for possession - you are not likely to be a party to the tenancy agreement, but it might depend on the wording of the documents.
I think you can only cease to be a guarantor if the tenancy ends, or if there is some other means specified in the documents.
These are my views but are based on limited knowledge - it seems you could possibly be liable for substantial sums so it might be worth taking all the relevant papers (including a copy of the tenancy agreements) to a solicitor for advice.