I will add in one thing ... if there is one colour that is extremely hard to get a match for, it is white! So painting the arch and sill with an off-the-shelf paint is 99.999% guaranteed to stand out and be noticable.
If the aim is to make it look tidier then it is currently, IMHO using hammerite is not the solution.
Best/Simplest DIY solution with a decent result is the black stonechip I would say.
PS. Talking wheels, I just noticed in the pictures of my stripey van, that also shows the before and after of steel wheel renovation. very ugly looking wheels before and then sandblasted and powercoated after.