Peter - they are just commercially available philips/posidriv plastic covers available on e-bay/amazon.
The screws had started to weather, and they are hard to 'restore', so I took the easy way out, with a little Bostick!
After taking a screw out it is clear that they are blackened, but the black coating isn't up to the job. So it is either cover them, as you did, or replace them with stainless.
My basic OCD personality means I'd always know that under the cover they were rusty, so I can't do it.
