Personally I would just leave them and enjoy the flowers as long as you can!

But if needs must, lift them!

However some people find Dahlias difficult to over winter so in my opinion the longer you can leave them in their natural environment the better.

This link from my website might help;
http://www.thegardenersalmanac.co.uk/Data/Dahlia/Dahlia.htm

If nothing else you can enjoy the pictures!

Thanks Teegee
Wish my dahlias look like the ones in the gallery of the link you sent me!

Following your advice and the advice from the gardenersalmanac, I've decided to leave them in situ for another couple of weeks - they do add such lovely colour to the garden. If the frost hasn't got them by then, I'll lift them. Last year I didn't get round to it and left them in place hoping they'd survive but I think they rotted away - that isn't going to happen this year!
Thanks for your response
Sonija

I got bored of mine yesterday and loped off all the leaves, leaving the stalks. Moth caterpillars and slugs where having a go at the leaves, so even though it had flowers I had to get rid. The wet weather has not helped, some of the tubers had rotted!

Glad to be of help!

BTW your comment;

"Following your advice and the advice from the gardenersalmanac"

is one and the same thing...its my website!

If the soil they are in is really wet, frost or not, they are going to start rotting. I lifted mine last weekend after a little snow fell (no frost, though) and they're in a cool, dry summerhouse for the moment, and I plan to put them in a boxful of dampish sand up in the loft overwinter when I have a moment. It worked well last year, but you shouldn't let them get completely dried out.

Thanks for your comments, Gattina and to all of you for your helpful advice.

For the sake of completeness, I'll just round off this thread by letting you know that following two fairly hard frosts my flowers started to look 'watery' and the buds were never going to come to anything this late, so I dug up my dahlia tubers. It has been a very wet summer, so I wasn't expecting a lot but both tubers were firm, fat and quite clean. Now I am going to follow your advice and see if I can over-winter them.

Many thanks again.