Phil, as it's under the car, You'll never see it unless you underneath - with that in mind, I'd use this stuff rather than hamerite underneath:
http://www.frost.co.uk/item_Detail.a...Paint%20(473ml)
It's as hard as nails, and will be more resiliant than the matt hamerite. Matt hamerite is great for touching up the odd ding or scratch in the powder coat topside though.