For every Sainsbury's delivery there is a third party delivery, approximately.
They don't deal with drivers effectively for the very reason you give: only if presented with incontrovertible evidence of an offence would they take action. This is an unacceptable way to run a business and flouts health and safety practice and their corporate responsibility. Part of the problem is talking about Sainsbury's as an entity. It is common (bad) practice for no individual to take responsibility within an institution for an issue, with the consequence that the issue is much more difficult to resolve. This practice is endemic at Sainsbury's.
Practical steps:
A few years ago Sainbury's used a member of staff to guide vehicles in from Pearcefield Avenue. This was very effective and, as far as I am aware, no incidents occurred when the practice was in operation. It was discontinued for no good reason and could be reinstituted today.