I went through the first 200 frames of the linear animation, fixing the walk and doing a bit of clean up as I go (which just me deleting frames that are no longer necessary or re-keying some things that have gone weird).
The walk is now about fixed. Won't do any more to it until the splines stage. Looking a lot better now, the feet are slipping about much less and the animation is generally a bit tidier than it was.
I'm going to wait until splines before I go any further with it because it's now about the timing and the slow in slow out principle, both of which would be easier when I can see the f-curves.
Moving onto the next 200 frames though and I have this little problem to fix:
I have no idea why this happens (I think it may be something to do with the way I move the joints while blocking - could be wrong) but this isn't the only time it's happened, there are a few 'spazz-outs' here and there. Should be fixable by deleting and re-keying some of the frames.