Thanks for all the advice! It's now decision time – heading off tomorrow morning…
I think taking both bikes isn't going to happen – I also have to find room in the car for my darling wife and children (and their bikes). And the daftest dog in the world. He *won't* be going on the trails.
Having just had a quick look on youtube, at least The Beast looks eminently doable.
If it's all completely miserable then I'll start by buying the fattest tyre I can find. If that doesn't work I can always hire a bike, or just go to the beach.
If you find yourself stuck behind some annoyingly niche fat old bloke on a bright red On-One Inbred picking his way painfully and slowly down the hill, you'll know I made the wrong choice.