I feel you may be being a little pessimistic with your interpretation of the figures from the survey. However, as I said before, I can completely understand your wanting to protect the hogs from potential harm and there is absolutely no reason why you should feel obliged to enter the hogs there onto the Big Hedgehog Map. It may be worth you contacting the Hedgehog Officer at PTES email@example.com suggesting a confidential database, possibly in tandem with the other one, for people who aren’t happy for the information to be more widely known. As you say, it could potentially give a more rounded view of hog numbers. Not sure how it could work, but I’m not very good at that sort of thing, so it may be possible.
The important thing is that you have hogs there – it sounds a lovely place for them. I am guessing that although you don’t have a hog highway, as such, that the hogs can actually access the other gardens in a similar way to how they access yours?
I hope the hogs stay safe (and especially keep away from that road) and that you have some happy hog watching this Summer.