Hello Tom and All,
There are two kinds of beekeepers:
"Those with resistant varroa and those going to get resistant varroa".
Further:
"With  overdosing and underdosing you still get resistant mites"
Also:
"Using according to label  instructions you still get resistant mites"
solution:
Alternate with a completely different chemical (which you across the pond
did not do and hence the problem !).
Quit placing blame and look for a solution!  Maybe a non chemical solution
of which there are many.
Sincerely,
Bob Harrison
Odessa, Missouri