I wouldn't expect that a minister would be involved in day-to-day procurement practices in a department. There are rules that state that when a procurement goes over a certain threshold, ministers need to be involved in the approval process, but in the day-to-day.... Most of the contracts we looked at over the two audits on professional services were just about day-to-day contracting.
I do think that a minister should ask the deputy head about how they are ensuring that rules are followed and that good value for money is always achieved. It comes down to those with that delegated authority within departments to be aware of the rules, to apply them and to know why it's important to achieve value for money.
Value for money doesn't always mean the lowest price: It means the best service or best product for the government at a competitive price.