....but I've just had to write a written proposal as to why our staff require risk assessment for driving. Not a proposal as to the degree of risk assessment; but risk assessment full stop. The "we've never done it before" excuse.

It is your employer's reputation that is at risk if they foul up, it is your employer's bank account that will take the hit if they foul up and your employer's additional cost if they lose an employee. Management Regs Reg 3: Every employer shall make a suitable and sufficient assessment of- (a) the risks to the health and safety of his employees to which they are exposed whilst they are at work. If your drivers are driving for work these are the financial and legal reasons.