Okay, well having spent a few days looking at various sites in various niches (especially the tough niches) I *think* I *might* have a solution...
...but accept that I also might not.
By way of an experiment to test this out I have taken an old site of mine that used to rank between 3rd & 10th on page 1 of G, and that disappeared out of the top 1000 on the 24th.
The home page still isn't in the top 1000, the site is an org uk and is related to the menopause, specifically to the symptoms of it
Hope that tells you what the site is, I obviously don't want to post it's URL here.
Anyway, i've made a few adjustments to see what happens, and I have a list of another 4 changes that I will make if there's no response within a week or so.
If the site comes back I'll tell you all what I've done.
Fingers crossed and all that.