Tomato plants wilted overnight

Mine looked like that, and it definitely wasn't underwatering. Twice a day.

 

The grey powdery stuff looks like tomato blight, might be worth browsing the forum to see some good close up pictures and descriptions of blight or get something out of your local library. Be careful if it is blight that you remove the infected plant before you buy any new ones. Hope this is of some use

 

It looks like you are using a porous membrame - this will not hold water. replant into a pot.

Compost looks a bit woody and again will not hold water.

How have you planted them as It looks like the soil has been banked up behind?

Its been so hot twice a day might not be enough - it depends on how much water you give them and you need to get it down to the roots.

Too early for blight and it sends the stems black

 

The plants are all planted in a raised bed, the membrane is porus and the soil beneath is heavy clay. The powdery stuff is ash, a added a bit from a bbq. The compost is a mix of 'good' stuff and a cheaper upper layer to add some bulk - it was going to cost about £80 to fill it with top quality compost, so there was a trade off - 70% good 30% cheap.

Digging down a couple of inches, the soil is moist and open - it looks perfect to me. I will post some more pictures asap.