Overwatering tomato plants?

I’ve been growing 9 tomato plants in tomarite grow bugs in my polytunnel (plum celine, gardeners’ delight and moneymaker varities). They have been doing really well and all have several trusses of green fruit on them (the plums especially are huge). The plant nearest the door has always been a bit yellower than the others, however this weekend I spotted that some of the stems are starting to turn brown and look like they’ve got greyish mould spores on them. This has started to spread slowly to some of the other plants and a couple of the tomatoes have dropped off.

I am using 3 litre plastic bottles with spouts poked into the growbags as slow release water reservoirs and watering them about 3 times a week, refilling the bottles a couple of times with each watering. I’ve also been alternating tomato liquid feed with a liquid manure / nettle feed every week.

What could be causing this? Do you think I’m over watering them?

I reduced my watering to every other day because to the weather

Thanks everyone. I will cut down on the watering (I only get up there at the weekends and once during the week  so I worry the poor things will get thirsty, but maybe I've been over zealous with the weather being as dull and wet as it has been of late).

Have you been shutting doors and windows up? - it's Botrytis cause by too damp an atmosphere.
Pick off all effected leaves/fruit and leave windows/doors open unless it's less than 12/13C outside.

Unless it's very hot the toms wouldn't want more than about a  litre a day each - but I'm sure your problem is not enough ventilation

I think you're probably right. I've not been opening the door much because it's been cooler. Do you think my chillis will be OK if I leave the door open a bit? They're quite near the door but there's nowhere else to put them. One of them's starting to go red, which is very exciting!! 

Do you think my chillis will be OK if I leave the door open a bit? They're quite near the door but there's nowhere else to put them. One of them's starting to go red, which is very exciting!! 

Hiya, As I am just up the road, I thought I would let you know that I have been keeping my door and window open on the greenhouse (despite it getting a bit colder) and my chillis are OK.