Non Matching Signature on Deed of Guarantee

Guarantor

Hi,
An interesting one this.

I usually try to meet any Guarantors but in this instance he is 300 miles away. I have received a copy of his passport and he has signed (And had witnessed)a Deed of Guarantee.

However, his signature on the Deed is nothing like the one on his passport, and I mean not remotely similar! Would this non matching signature be valid because of the witness verification or should I ensure that a new document is signed with a signature more closely matching his ID?

I somehow doubt a bank would accept a signature so very different from any ID provided.