How can I get my tomatoes to turn red?

My tomatoes have done very well, producing a healthy looking crop:

But although it's been quite warm and sunny, they've been reluctant to turn red: 

So I was pleased to hear someone on Thought for the Day say that if you put tomatoes in a paper bag with a banana, they'll ripen because the banana gives off ethylene, which is what makes tomatoes turn red. 

I'm not sure what the lesson about life was supposed to be. Maybe something about just because you shouldn't put all your eggs in one basket, that doesn't mean you can't put all your tomatoes into a bag, as long as you remember to include a banana.

Well I tried it, with mixed results: one beautifully ripened tomato, one partially ripened, and one that rotted: 

But this was better than leaving them on the window-sill to ripen, which didn't work: 

In fact, there are some more going red quite well outside now, so I'm just going to leave them there unless a frost is forecast, in which case I'll put them into a cardboard box (another idea, off the internet), maybe adding a banana for luck.