Mottled Yellow and Brown Tomato Leaves

As you said, it could be sun scorch

I have to second that, as its the leaves most vulnerable to the heat that are affected, it seems the ones lower down in the shade are ok.

I know a few people on hear will preach about water droplets on leaves in the sun causing scorch, well I just try to base my advice on how things have worked out for me.

I spray my toms with water 3 times a day, if I am around on very hot days, I also wet the plastic and the wood in my own poly tunnel and damp down the floor a little. I have no real scorch problems, even though my leaves are very wet in high sunlight.
My theory is that when t is very hot, the plants cant take up enough moisture to cool themselves down, especially in pots or bags where they have a smaller root system, so I make the air very humid, and also cool down the leaves at the same time.
The plants however must be totally dry by night, and the doors left open so the humidity drops as it cools down.

The shade you are putting is a real help, I havent even got any! as the bloody weather keeps changing so much!

Sorry for long rant , but I am away from home stuck in a hotel room and the tv is crap, LOL

Thanks All
I have plastered the GH with windowlene today to try and aid the shading. Im paranoid as this is the first year with the GH and im doing things properly for once. (well I think im doing things properly )
In past years I would have some tomatoes in buckets next to the house. They got tomrite once if they were lucky and normally ended up getting bashed to the floor by the wind. I swore every year I would never grow them again as I ended up with loads of green toms that I could never get ripe.

Fingers crossed I will get a decent crop of ripe toms for the first time ever this year.