Guarantor Refusing to Disclose Income

Guarantor

Have 3 Students proposing to rent from us August 2014, each have nominated a parent as a Guarantor, all are home owners.
Two of the Guarantors are not happy about disclosing their income. One of the Guarantors is Retired their income is from holiday lets. The other is self employed. The Retired Guarantor has offered to pay the years rent in advance, search application against this Guarantor has been declined due to income not being submitted. Still awaiting results for the remainder Tenants and Guarantors.
One of Students, who's Guarantor wants to pay in advance, I know his present Landlord who has confirmed rent is paid on time etc.
Not sure whether to cancel the whole application and readvertise, feel as if its getting messy!!
If the remaining searches are approved, do you think it would be safe to proceed providing full years rent is paid in advance by this one Student, the other two pay as normal monthly and when should they pay upfront, as concerned that once AST Agreement is signed (normally around about May 14 - Tenancy starts Aug 14)they decide not pay upfront.
Any help would be appreciated.

Thanks.