Mrs. Figg's 'Cat among the Pixies'

[lizvega2]

No: HPFGUIDX 93516

In OOP, A Peck of Owls, P. 24, Mrs. Figg says, while commenting that 
DD told them to make sure that Harry didn't use magic, that "Well, 
it's no good crying over spilled potion, I suppose...but the cat's 
among the pixies now..." 

Now, the first part of her speech, 'there's no use crying over 
spilled potion'- I understand, but the latter part about the 
pixies..Is that merely an extension of the former? Is this an 
English saying? Does it say the same thing in the British version?

LizVega~forgive me, I'm American. :)