A questionnaire is one method and legitimate method for BRC. However...
If so does anyone have any ideas as to what I would include in this questionnaire, and really is it sufficient?
In principle, the higher the risk, the more requirements to "assure" one of a supplier's safety and quality. Risk can be based upon a number factors (food/ingredient type, history of food and the supplier recalls in the past (if any), their processing method, your further processing method, environmental factors, etc).
The key here is to develop a method to reduce exposure (like the retailers are trying to do) to food safety and quality events (and their life threatening and/or financial costs).
For example, if one is processing vegetables, think about Taylor Farms supplying you spinach (and their recent recall history and the end R-T-E product) vs. Simplot raw potatoes that will be further processed with a kill step. One will be a different risk than the other, one should require more to "assure" you of their safety than the other. These risks would guide the questions you would ask (the former just may be too risky for an questionnaire approval than the latter).
I hope this helps some.
Cheers