1. I would have thought the guarantor details would be in the tenancy agreement, of which the agent must give you a copy.
2. So I presume that, for some reason, they are not in the agreement. That leads to 2 possibilities:
a) There is no guarantor and the agent is misleading you; or
b) The agent is genuine & has some valid reason for not giving you the details - although I cannot imagine what that might be.
2. You need to point out to the agent (in writing, keep a copy) that you have a vaild reason to know the guarantor information - i.e. to reassure you that the rent can be paid if the tenant defaults - & ask them to confirm to you in writing that there is a guarantee in place and say precisely why they will not give you the details. If you are not satisfied with the reply you need to write to them again holding them responsible for any rent not paid by the tenant if there is in fact no guarantee in place. Then change your agent as soon as possible.