Thank you so much to everyone who replied, every response has been so helpful with my current research.
My grandfather was only 2 years old when his father died so would have had no reliable memory of what type of vehicle was involved unless there were photographs which I would have thought unlikely given their circumstances.
I also agree that the description given on my grandfather's marriage certificate should be taken with a pinch of salt, especially as at that time he was knowingly or unknowingly claiming to be four years younger than he actually was (same day and month but an earlier year).
So I guess where this leaves me is that whilst my great-grandfather drove a horse and cart for a living, whether his cart was enclosed or open, whether he had progressed from horse to engine in the years between the 1901 census and his death in 1909, or whether his occupation was simply being described a little differently in 1937 remain unknown.
Thanks again to everyone who has taken the time to respond
Best wishes
Stephen