I have blight on my polytunnel tomatoes. Should I clear the lot?

RHS advice is to destroy any infected tomato plants and not to compost them

If the situation is already this bad I don't see what you have to lose by leaving the remaining plants. Just cut off diseased leaves as soon as you notice them. With a bit of luck you might get some edible fruit. Last year when we had blight (thankfully this year not a sign of it, as the weather in France has been hot and sunny!) we found the cherry tomatoes remained edible even when the plants were covered in blight. Unfortunately the larger fruit all went brown and were inedible.
Very sorry to hear of your bad luck. It's so dispiriting when you have waited all year to get your tomato crop and this happens.

Thanks Bertiefox. I had been considering what you've suggested. What is there to loose?

I do hope it works for you. A 1/3 of my tomatoes plants were showing signs of blight and I cleared them hoping the remainder would be fine, but all went down very quickly (within a week!) and I had to remove all of them. Good luck.

Bertiefox, I would be concerned about leaving them in place in case the blight spread to potatoes

This is a good website on treatment of tomato blight

http://www.associatedcontent.com/article/286560/how_to_treat_tomato_blight.html?cat=32