A good few things to think about there - Thanks chaps!

Of course, one thing I didn't mention is that these stairs are open
(no risers), and the treads sit slightly above the strings, which
makes it all a little more complicated(!)

I guess if I route (that doesn't look right, but I guess that's what
you do with a router!) as far as I can then get a carpet to fit that
width (the shops seem to only want to sell cut carpet and the old-type
stair runners), it should work.

Then it'll be a matter of how to tart the edges up - Looks like I'll
be learning a lot about restoring hardwood in the near future.....