Dahlia - help needed after bloom

Attached are two photos of a recently purchased Dahlia 'Roxy'. New blooms are appearing all the time but the petals of two blooms have fallen off leaving the head with the yellow center, now closed up (see photo). My question is what do I do now with the 'finished' bud? Do I just cut off the head at the end of the stalk or do I cut the stalk off at the base. Or do I leave the whole thing as is? Also I wouldn't mind advice as to what to do with the plant when its finished flowering altogether.

 

It's usual to cut at the first leaf Charlie, I'd chop it personally but I might be wrong. Ooh I am harsh!

 

I'd just dead head it Charlie, dont cut too low, just to the nearest joint.
Once its finished flowering its normal to let the frost kill the top growth, then dry it off and store the tubers frost free.
But thats a long way away, dont start thinking about that ALREADY,

 

Thanks all

 

Oops that's what I meant, first leaf down not first leaf up!! Lol!