I think this will be what is called a holding deposit, rather than a deposit related to renting a specific property. Sometimes these deposits are said to be non-returnable.
Did you sign anything related to the deposit? If so, look at the small print & see what it says. If it says non-returnable you might have a problem because it may not be easy to prove that the agent misled you about the guarantor's status. However, the form you completed should have included a question asking whether the guarantor was a home owner & you could point out that the absence of this question was a complete failure of openness by the agent.
If the agent firm belongs to a trade body you might find they have a disputes procedure you can use as an alternative to a Court claim.