Courgettes - yellow spots on some leaves of some plants

Some of my courgette plants are developing yellow spots and patches on some leaves, please see attached photos.

Any ideas of a cause please and a dog-friendly cure? (I don't want to use conventional pesticides for various reasons, including the possibility of my dogs coming into contact with anything harmful via the leaves, etc).

Note: My plants are a mix of various varieties and I do notice that the Golden Zucchini variety generally have much yellower leaves as they age and grow larger, see my Spots-3 pic (smaller, younger leaves far less so), however as can be seen in the photos one in particular definitely seems to have some kind of spotty issue (although I don't recall which variety that is - notice that there's also slug damage on the worst affected leaves).

Thanks

 

Suggest you check underleaf surfaces for signs of aphid. Did you grow these in a greenhouse where there might be greenfly? (larva).

 

Thanks, just checked and not an aphid in sight.

Not grown in a greenhouse, started in my house then moved outside a few weeks ago. No signs of greenfly in my house (no other plants either).

 

I've seen those yellow spots before but normally towards the end of the season.
I think the last picture is something different, possibly red spider mites.
Or scorch.

 

Probably wont help much but
have a look at this:
www.vegetables.cornell.edu/pest-management/disease-factsheets/virus-diseases-of-cucurbits/