Help with prospects of recovery of unpaid rent

Guarantor

My letting agent (an ARLA member firm) found me a tenant who they informed me was a high level self employed business broker with variable income. Because of the variability of the income a wealthy family guarantor was available and willing to sign the deed of indemnity (the Deed).
My agent sent the guarantor the Deed by fax, the guarantor signed it, faxed it back to the letting agent who then signed it as witness (obviously not actually witnessing the signature). I was unaware of the manner of ‘witnessing’ and assumed the deed had been properly executed when I signed the tenancy agreement.
I assumed ID verification of the tenant had been carried out, but it had not and it subsequently came to light that the tenant had recently changed his name by deed poll, was a recent bankrupt and was on Income Support at the time of signing the tenancy agreement.
The tenant has not paid and I now have a court judgement on the unpaid rent (around £9000). The tenant is now in prison for crimes unrelated to the tenancy. There is no point attempting to enforce the judgement – the tenant has no funds.
The guarantor refuses to pay stating she was under undue influence to sign the Deed and that the Deed was not properly executed. The agent did not make any contemporaneous records at the time of dealing with the tenant or the guarantor. The prospect of building a successful case against the guarantor is therefore compromised.
The agent says this is business risk and my loss of rent and all the trouble I have had is not their responsibility
Help.