Possibility: Pepper mottle virus. Could have been from nursery. It is often insect transmitted and some nurseries do a poor job of isolating and caring for their plants. https://plantpath.ifas.ufl.edu/plant-virus-profiles/ssrnaplus/Pepper%20mottle%20virus/index.html

https://www.seminis-us.com/resources/disease-guides/pepper-eggplant/pepper-mottle/

Bacterial leaf spot. You can control with chemicals if you want, but if you’ve never had it before I would honestly recommend bagging and binning asap to minimise the chances of recurrence next year. Otherwise it will most likely infect subsequent crops no matter how well you control.

Thanks you two. After reading various websites I think it matches the symptoms of bacterial leaf spot fairly well. Just what I needed, another disease to deal with while I'm already in defense mode. So jealous of people who just plant stuff and it grows!

You did the right thing by quarantining them in pots for a few weeks. It is pretty unlikely that they were infected at the nursery because the disease develops pretty quickly; you would have seen the signs within that quarantine period.

Note that it also infects tomato if you or a neighbour grow any of that.

I feel your pain. Good luck.

I'm already in a rotation regime with my vegetable beds because of what I believe is fusarium wilt, so peppers and tomatoes should be several years apart once it is fully up and running. In this case the infected peppers are in a bed that had tomatoes last year, but nothing much before that for several years (it was a defunct strawberry bed and essentially fallow).