[bailey]

as I have had to re-cover my polytunnel I have thought about digging a shallow trench, or area, placing some poly sheet then maybe a covering of gravel, then place the plants on this, as they are in pots,, anyprobs with this??

I already do this at home with toms in a cold frame, with the inside of the frame covered in black poly sheet the pots sit in about half an inch of water,, seems to work up till now but not too sure when they start to mature, always been told to water from bottom and feed from the top.

[Jay The Digger]

Sounds like a good idea for Peppers and Chilli's, as they thrive in a nice humid atmosphere, but your tomatoes will suffer in this environment.  Even if the pool of water is covered by gravel it will get quite steamy in the full summer sun.

A warm humid atmosphere is where blight thrives.

[sunshineband]

You could maybe try growing your toms nearer the open end, and grow them in rings, with compost in the ring and water roots in the gravel 

[8doubles]

Toms will be ok if ventilated as they drink a lot of water on a sunny day.
Chillies hate having their feet wet all the time and will die if overwatered. I never water my chillies till the leaves wilt because of dryness.