I'm afraid this sounds horribly like (potato/tomato) blight to me. If there are brown marks on the stems too, then it is probably too late to do anything. I imagine these are outdoor rather than greenhouse tomatoes.
All you can do is to remove the damaged leaves and fruit, and then copiously spray the healthy growth that remains with Bordeaux mixture. I have found this has helped the problem in previous years and still allowed me to harvest some of the crop.
Has it been wet and humid in Cornwall recently?
If this problem occurs in most years, spray the plants with a copper based fungicide like Bordeaux mixture as a preventative.

Yes they are outdoor plants and yes it has been wet and humid for the last week. They were doing really well up until now. I have removed some of the stems with the shrivelled leaves so is it ok to cut all the affected leaves? Many thanks.

You will need to remove the complete tomato plants Mistral and either burn or put them in your wheelie bin. Don't grow tomatoes potatoes or any other members of the solanum family in that land for at least seven years as the blight will remain in the soil.

Many thanks.

Less than fun thing to happen - and a belated welcome to GoY

Moongrower, I did find that spraying plants with BM having removed all diseased plants did help me get a healthy, if reduced, crop, though admittedly I remember the weather turned very hot and sunny after the initial disease, which may not happen very often in the UK.
Sympathies too to Mistral. I know just how heartbreaking it is after so much effort to have tomatoes suddenly afflicted by blight before you get the crop.

Many thanks for all your comments and the welcome. Yes it is soul destroying as I have such a bumper crop and really good size. If the tomatoes do not have brown on them can we eat them (although still green at the moment) or make chutney do you know please? This is such a lovely site. I have been away for the weekend and I received -emails with your very prompt comments. Thank you to you both.

I heard or read somewhere recently that you can gather green tomatoes from blighted plants, put them in an incubator that can generate at least 40C, leave them for 12 hours, and then take them out. The high temperatures are supposed to kill the blight spores and enable the tomatoes to ripen normally. Otherwise if you do have blight, the tomatoes will go brown during the ripening process.
Worth experimenting with the heat treatment. The article suggested using an egg incubator but maybe putting them in a glass frame in the sun would generate high enough temperatures.

Excellent, thank you.

To be honest I would be reluctant to eat tomatoes sprayed with BM organic it might be nice it isn't.