Hi Georgie6, if you already have hogs visiting your garden for food, then there's still time for a hedgehog to use your new hog house, but there's no guarantee it will be used for hibernation. Indeed, of the 6 hog houses I have in my garden, only 1 is currently being used! Ensure your hog house is placed somewhere safe and hidden and protected from the elements (e.g. under a hedge, shrub, etc). To lure a hedgehog into your new house, initially leave a small handful of food that won't be attractive to cats, such as chopped unsalted peanuts or dried mealworms. Once you know you have a hog residing in the house, stop placing food right next to the house to avoid attracting any other unwanted visitors, so hoggy can feel safe and sleep soundly. Above all, don't be tempted to keep checking the house, your scent may deter them. Instead, place a twig/similar across the entrance to the house and simply monitor if the twig has been pushed out of the way.
Large males tend to hibernate first, followed by the larger females (many of which will have reared an autumn litter, so will be busy fattening up now), then last but not least the smaller hogs/juveniles will follow - these are the ones who need help with supplementary feeding in order to reach a safe weight of 600g. Of course, some hogs don't hibernate at all, while others will periodically wake up in search of food/water.
I still had hogs visiting my garden well into December last year, then it all went quiet in January right through until late April.
Good luck with your mission, Georgie, I hope you're successful
EDIT: Don't know if you have already done this, but line the hog house with a wad of newspaper and stuff to the brim with fresh hay/straw and dried leaves - the smell of fresh bedding should also act as an attractant for a hedgehog.