one of the main problems is uneven watering -- either when the compost dries right out or-- as happened to me last year-- if the plants can't take up the water quickly enough when its hot
it can also be caused by an imbalance of nutrients
try keeping the atmosphere in the greenhouse 'bouyant' by hosing down the floor when its very warm ( but not just before shutting for the night or if its cold and damp outside) I even spray the plants a couple of times a day if its hot but shady but mine are in grobags and I water when the compost just starts to need it,
I'd give them a dose of epsom salts and if possible shade the greenhouse if they don't improve, I'd remove the affected tomatoes and just let the plants continue to grow
its been a funny year for hot one minute-- cold the next -- I think its got something to do withit!

I dont tend to remove them as the rest ripens and once the brown bit cut off i use them for cooking.
I was told it was uneven watering.