I think the issue is that cycling on that area (away from bridleways of course) is by permission only. If the Crown Estate wanted to stop it they could in an instant – just declare it off limits to cyclists – and while there would still be the potential to ‘nick some cheeky trails’ that becomes a bigger offence that affects us all.
It’s also the liability issue. If trails are in there and are publicised to an extent, then i guess there could always be a chance that if someone hits a load of walkers, or injures themselves badly and tried to sue the land owners, however tenuous or unlikely, there’s a problem created there as well. The permit buys you liability insurance very cheaply to cover this.
Fuller details here: http://www.gorrick.com/swinley/permits.php
Bottom line. You probably are unlikely to get ‘caught’ and even if they do what can they do? Ultimately, they could ban cycling from the forest. So this needs to be self policing. There is a great resource there, make it your business to have a permit and also make sure your riding mates do too. It’s in all of our interests to be responsible and for the sake of £2, or £18 for the year – it’s not a lot to ask is it?
[sorry – late edit]
And if you do see someone without a permit remind them they need one. You don’t have to aggressively ‘police’ it, many’s the time I’ve reminded riders they need one only for them to say they weren’t aware of it. Sure, you get a few that tell you to sod off and stop bothering them, but they’re just c0cks, aren’t they.