Well, the plot thickens... I went out to look at it last night (in the dark, of course!), and noticed that the right-side plastic step pad was a bit lose. In trying to look to see what was under it, it popped off completely!
I had an accident a while back and the body shop replaced the bumper skin, and if I recall correctly they re-used the step pads and just replaced the bumper. (I was paying, not insurance).
So they glued them back on with some black goop, which is now letting go.
So now I have even more incentive!
Looking at it, I think I could leave them off completely and put the Diamond plate in their place - I was originally thinking it could go on top of them.
I think it would be cool to wrap the diamond plate over the edge like the originals do, but there is a slight curve so that could be tricky! I'm not particularily skilled nor do I have any equipment for this kind of metal working but I know aluminum is pretty easy to work with...
OK have I gone on long enough?

I'll take a couple of pics in a bit and post them up.
Cheers,
Dave