I can't contribute to the discussion on recent trends in the UK because I've been out of the country for too long. In my youth they were recovering (successfully) from Dutch elm disease but I also know they are periodically subject to crippling parasitic infections and in some years a great many eggs are apparently affected.
I did go out looking for WLH eggs this morning but suddenly discovered I couldn't identify wych elm in the snow, when there were no leaves to be found.
In the summer I found several young wych elm trees/bushes growing by the path but I didn't note exactly where. I have had the same problem with honeysuckle, looking for white admiral hibernacula - I simply can't identify the bush, among the dozens of other bushes present. HOWEVER, I did check out a few oaks while walking home and found this purple hairstreak egg lurking not 200m from my house:
This was good news because I've never seen the adults around here (I only moved in a year ago, though).
For this species, which lays consistently in easily accessible and predictable places (south side of young oaks, isolated or on edge of woods, at accessible heights) I think winter egg hunting must be a very good way of finding where it flies.
Guy
