In reply to FactorXXX:
Section 34 of the Road Traffic Act 1988 makes it an offence to drive a motorised vehicle off the road:
(a) on to or upon any common land, moorland or land of any other description, not being land forming part of a road, or
(b) on any road being a footpath, bridleway or restricted byway.
There are various exceptions such as being allowed to go 15 yards to park or if you can show it was an emergency.
Obviously land owners such as farmers can do what they want and can give others permission to do what they want but if permission was given for someone to "lead" a group of motorbikes off road on a regular basis then I suspect that you might need planning permission for change of use which I doubt would be forthcoming in a National Park.
I have in the last couple of years observed off road bikes being ridden over the Silverflowe in the Galloway Hills which is a floating bog and SSSI, up High Pike in the Northern Fells, and more recently up Ennerdale towards Blacksail Youth Hostel. As mentioned above, these bikes tend not to have number plates on so there is not much the authorities can do with a report. The only thing that one can do is if one sees people loading or unloading trail bikes from a van, try and get a few photos and the van number plate and report them to the police. If they are riding them on the road they are breaking the law and if they are riding them off road, ditto, so the police should, at the very least be able to "have a word".
The one exception to all this are rough tracks that are officially roads (like the Walna Scar Track near Coniston), so you can ride this on a motorbike, but you need tax, insurance, number plates and a licence.
I'm a biker myself and love riding my bike but I don't think they have any place in our wild mountain areas.