There are a few 'loopholes' you could try. Did you have sight of the AST (the tenancy agreement) before signing the guarantor agreement? If you didn't, or didn't have a reasonable length of time to read it (I mean several days here, enough time to get legal advice), then the guarantor agreement is probably legally unenforceable as you would not be aware of what you were guaranteeing. Ideally, from the LL/Agents point of view, you would have signed a copy of the AST along with the tenant and LL so that you were aware of all the Terms.
You say that the tenant re-signed a new agreement every six months? Did you have sight of each of those agreements? I would doubt if you did. If you didn't, again the guarantee probably wouldn't be enforceable as the Terms of the AST could have varied considerably (not just the rent) and you wouldn't be aware of them.
By the way, a guarantor is liable for not just the rent but all other costs that the tenant may be liable for too e.g. damage at the end of the tenancy, breakages etc during the tenancy, LL's legal costs to evict if it came to that.
Getting someone to stand as a guarantor for a tenant is great for a LL as it gives them a fall back position if the tenant defaults but I personally would never do it if asked.
I'd suggest if you want more detailed advice then try the Landlordzone Forums
http://www.landlordzo.../forumdisplay.php?f=3