Aaargghhh! I did stuff it up - I made my nice clean washes into mud and murky browns by putting too strong darks too soon and then making it worse worrying at it instead of going away and letting it dry. Nothing like displaying one's failures to all and sundry (I hope you all learn from this!)
After I finally let it rest and dry, I lifted some of the murk and at last followed Laura's sensible suggestion (see comments on previous post) to work a bit more on the eggshells before going back to those #*!* shadows. If nothing else this work-in-progress exercise will serve to remind me what not to do in future. I will take Jana's serene advice in her interview with Liz Massey to learn from the experience, let it go and move on. But I will keep on at this for a while longer, until I've either rescued it, or am totally satisfied that I've utterly botched it.