On reflection, and given the very helpful and constructive criticism from Paul and Iain, I've gone back and had a second go at rusting a bit of rail and girder. While the powder based approach might work well for rail dumped in a bog for a few years I admit that it wasn't so good at representing rusting surfaces which tend to discolour without such serious pitting.
On this second attempt I again started with red oxide primer and then started to add shading using the same paints as before. At this stage I wasn't entirely convinced that I was getting anywhere but the addition of some weathering powders (rust and dark earth) seems to have pulled things together nicely. Not only does the texture look more convincing but I think the colour is better as well. Hopefully you all agree?
This episode reinforces the main reason I enjoy blogging and getting comments; without Paul and Iain's feedback I may well have just continued with my first approach and the final model would have suffered as a result.