If you are up to 29 minutes late you will probably be fine but, if longer, it is likely you will be charged for an extra day (Van rental companies charge in 24 hour intervals) at the normal daily rate that is charged by your supplier (not the discounted rate you will have received through MoneyMaxim).
If you think you're in any danger of running late, a quick courtesy call to the rental company to keep them informed is certainly in order. It won't guarantee that you will avoid additional charges (although if the rental firm are feeling generous it may do!) but advance notice can help them to avoid disappointing another customer.