I have always wanted to go into this tucked-away garden. You can glimpse only this, but it promises so much more. I imagine a really interesting garden with lovely little surprises all around. One day perhaps I will knock...
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I can assure you that most gardeners are more than willing to give you a tour - a long tour! :) I say knock, and if you offer to send some free pictures you'll probably be even more welcome. Unless the gardener is a hermit in which case you're out of luck.
I have done the same--admired a garden and shot from afar. If you do knock, show them this photo tell them you just want to capture more beauty. How could they say no to that?