Thanks again to E. from the Twosie The Hedgehog blog for helping me get this little fellow delivered safely to Tiggywinkles wildlife hospital yesterday.
The staff at Tiggywinkles told us that they are now quoting a safe hibernation weight of 500g for a hedgehog rather than the previously quoted 600g, possibly because of the milder weather recently. The little one we took in (who doesn't actually look so small in the photo, but he is all stretched out) weighed approximately 470g so he was a real borderline case. At that weight, i.e. over 400g, they said technically he's not a juvenile. However, they did say to bring him in when I telephoned and so that is what we did.
And yes, it turns out that he is indeed a boy.
I asked what would happen to him and was told that they would fatten him up and then when he reaches the required weight they would probably put him outside in the Tiggywinkles grounds, all the while keeping an eye on his progress. Then all going well, he will be released into the wild in the spring.
Thanks again to both S. and E. from Twosie the Hedgehog for all their help, advice and kindness, and for the above photo!