Brown spotting on tomato plants

Hi everyone 

I've been growing tomato plants all my life and never had any problems, but now I have.

I've got brown spotting and yellowing leaves appearing on my tomato plants I'm not sure what this is.

I've had the greenhouse completely shaded during the hot weather, all lower vents and doors left open so there's been plenty of ventilation,watering has been done via a drip irrigation system everyday with the usual tomato feed done by hand.

I've started picking fruit which are clean and disease free.

Can anybody give me some advise on what this problem might be and is there a cure

Many thanks 

Brian