Help - cucumber problems

Had same issue last year .... to eliminate plants this year Cucumbers grown from seed (F1 mini)
All starts well ... and getting loads of fruit - so that side is fine.

Issue is the leaves ...
As plants get bigger .. lower leaves start getting discolouration patches ... it spreads almost to extent of leaf fully covered.

What is it and what do I do about it.

At first I though Magnesium deficiency as arrived at same time fruit started appearing - so sprayed with Epsom slats solution.

My searching seems to point to Mosaic virus - but can find no treatment

 

Have you looked very closely on the underside of the leaves for red spider mite?

 

I just had a good search with a magnifying glass - I can see no bugs or mites

 

If it is a virus, there will be no cure.