Obviously I don't know your house situation when you mention 'back door open'. However I do know that my little cocker wouldn't have the confidence to go off into the garden on her own, we have an upside down house so it's quite along way from me in kitchen to the garden IYSWIM! Uliss definately likes to be taken.
Since Uliss was speyed she does drink alot more and when she has to 'go' I have to drop everything and take her out, she does ask me and therefore I feel it is my fault if accidents do happen. The trick is to understanding her "ask", Uliss pokes me on the back of the knee and runs around in circles!!
I would have a look at some foods recommended on here, there are lots of recommendations, and try taking her out into the garden every half hour after she has been drinking alot. Uliss drinks alot after eating whatever food I feed her. After Uliss has her dinner at 4pm she goes a small walk, then she will need to pee at about 6'ish, 7'ish and finally before going to bed - just to give you an idea of our routine!!
Best of luck.
