Jon, POR works well on rust or rough surfaces, on smooth or even clean steel is does as you say peel off in sheets ! See you Sunday.
James, never been a fan of Hammerite, find it rather brittle, chassis black was my first thought, tho came across the POR Hardnose that if the blurb can be believed should work well. (http://www.frost.co.uk/por15-hardnose-paint-grey-946ml.html) Shame you can't make Sunday.