Interesting, thanks again folks.

A little update on the situation and a possible twist in the tale.

I've been measuring the PH only by using a cheap aquariam tester kit which is absolutely fine for readings between 6.0 and 7.6.
Now since its hydro I've been aiming for about 5.5.
My tapwater has a PH of around 8.0 so I needed to lower it significantly by adding the PH down.
The tester kit comes with a card with colour matches for each PH level with a pale yellow being 6.0. Since I was aiming for 5.5 I thought that I should be looking for a darker yellow but not quite orange colour based on full PH kits I looked at on the internet.

This is where I believe I have messed up!
I am an idiot!
Since reading this I've been investigating the situation further and suddenly realised that the PH indicator might not even change colour below pale yellow so I tested the babies water with a very acidic solution and there was no difference!!
Pale yellow is as yellow as it gets so what I thought was a PH of 5.5 could actually be as low as 4.5.
I need a real PH tester but I can't do that right now unfortunately.

So, could it be too acidic that its making the leaves curl up just cause its acid or could I be locking out some nutes because the PH is too low?
Would it cause leaf curl?

Stinky, I'm still not convinced its heat because some smaller plants have it too (they are 4 inches further away than that big one) and the air temp/circulation/ventilation/humidity are all fine.

Anyway, it was time for a water change so they have a fresh reservoir with full strength veging nutes with a PH of just less than pale yellow which should be roughly where I need it to be this time. At least it won't be pure acid anyway!

On the way I have fed my babies, I don't think it is a problem.
I grew lots of plants years ago with a 3 part and as soon as they went into full veg under the HID they were whacked upto half strength then full strength a few days later and I never had any problems!
This time I've gone easy on them by putting in the 1/4 strength step and spreading it out a bit longer.

Cheers again and I'll let you know how they get on but I feel that a stupid mistake like this must surely be the reason for the abnormality.