Dahlia Tubers

The old advice was to wait for frost to blacken the foliage, then to lift, dry off and store for winter. If your soil is well drained and you do not get 'deep' frosts then you can get away with leaving them in the ground. You could mulch over the tops as added protection if you do decide not to lift them.
Will just add, that we cannot even grow them at all here, the new growth is blasted by late freezing winds and then those that survive often get frosted off before they flower.

You are just down the road from us, I will leave my Dahlia tubers in the ground. Still lots of flowers to come. After a hard frost I will cut them down to the last leaf joint, put slug pellets down. Mulch with 4 inches of compost.

I'm in Berkshire and I leave most of mine in too and do just the same as DrB.

I;m in West London - the only one I lost last year was the one I dug up and potted in sand/peat and left in a frost free place! I won't be lifting any from now on...

very wet winters bad dry cold winters grate , mostly leave them in, you can do a lot of work if you lift them to do it as per book and you stand more chanch of looseing them than leaving them in the ground,

They are in a trough sort of pot - and I think they would get waterlogged if I left them outside. I think I will leave them in the trough, but move into the greenhouse - does that sound like a good plan??
Doctorbob - cutting them up to the first leaf joint is still a good 4/5 inches of stem for me - is that still ok?