brown sunken spots on immature fruit

It sounds like "Blight" as of Potato blight and there is no real cure.
You used to be able to spray with Dithane when ever you saw the first signs but don't think this is available any more in the UK. Once it is established - the leaves usually show brown edges first- then the whole plant should be removed and destroyed ...burnt preferably or your whole crop will be affected. Wet conditions are the cause - so don't water in the evenings and close up your greenhouse...that will give the blight perfect conditions!!!

If they are in the greenhouse, unless you are unlucky, they should have escaped blight which is a fungal spore and is carried on the wind, though it can also remain in the soil (hence the need to clear up leaves from potatoes and tomatoes and not to let potatoes sprout from old tubers still in the ground.) If you have blight, the leaves of the plant will certainly be affected before the fruit, so you should have noticed brown concentric rings on the leaves. You might be able to save the situation by removing any remaining diseased leaves and spraying with Bordeaux mixture, or any other copper based fungicide which you can find. But wash the fruit well if you do this before consuming.