The guarantor policy of the different lenders vary greatly.
Some lenders will need the guarantor to have a minimum income, some will look at net worth, while others require the guarantors net income to be strong enough to make up the mortgage shortfall.
As for the amount of the shortfall allowable, again this varies from bank to bank and is usually on a case by case basis.
Life cover is usually required for the guarantor but again it's case by case.
Finally most lenders have a maximum age of 60 but some have, you guessed it, a case by case policy.
Basically if the guarantor is extremely strong the bank will be happy to do all it can to please the guarantor, if the guarantor is borderline then the mortgage is likely to be declined.