Withdrawing from a tenancy guarantor

[Chezsue]

The terms and conditions state ' that the gaurantor's obligations shall apply both during the term granted in the tenancy agreement and during any continuation or renewal or variation thereof.'

I stupidly signed to this agreement to help a student taking a one year course. He has finished his course and I no longer wish to be his guarantor.

Is this not the case that the Tenancy Agreement has now become periodic, and that I can now opt out of the deal by writing to the landlord to terminate the agreement?
Also can a legal contract state a term which says that another party is bound to the contract even though the terms have gone through a variation without the other party being informed of what the variation is and whether they agree to be bound by such terms.
If this is not legal, does this make the whole contract void or voidable?

[another-view]

You agreed to "...or variation thereof."

You agreed to this. I don't see what you can complain about. It is important that you actually READ what you are signing. If you disagreed with it, you should have said so then, not now.

[bednobs]

no it is not unenforceable

[Mr Veritas]

You would be able to opt out of it, but for the fact that you signed a document containing as part of it these words:~

"during any continuation or renewal or variation thereof."

by signing under these words you have tied yourself in as Guarantor.
Sorry.'