No idea Owdb. but curious to find out...

Alternaria? This is a guess Boggy and it still looks like it is in it's early stages. It is usually caused by magnesium deficiency which causes a chlorophyll breakdown which creates spots and blotches in the leaves. It is apparently most prevalent (but still fairly rare) in tomatoes and potatoes. Not much data is available on this subject but you could try googling it to see what you can find. If no conclusive evidence is available, I would try an epsom salts feed if you are stuck. May do no good (as I said, I am in the dark myself never having seen this) but, apart from the cost of the epsom salts, it can do no harm and if you are stuck, may help.

Thanks. I have looked up Alternaria and none of the images look like what we have here. But then neither do any of the Virus images. Very odd.

It looks like blister aphid but I wasn't aware these were known to attack members of the solanaceae, usually they attack currants. Can you see any aphids on the leaves?

First thing we checked, even under a magnifying glass there is no sign of insect activity.
No one seems to know, though Oedema seems to be the favourite at the moment.

In that case it might be a mosaic virus, see-

http://cipotato.org/publications/pdf/002408.pdf

The virus is only in non-certified seed which these definitely were. Also, there is no mottling which gives the Virus its name.
Really am coming to the conclusion it is some form of Oedema caused by the high humidity in the tunnel.

Now you've suggested it Boggy, I reckon that is the most likely.