Unfortunately, you cant go off-road on the moors..you can only use the roads going through them.
There are very few lanes in Cornwall now.....most being classed as footpaths or bridleways, although the Surf boys have a couple way down west.
Main place is Baldhu nearTruro, you can go anywhere as its unregulated, but beware of DEEP water and LOADS of TREES!!
There are some pictures of the site in this section.
viewtopic.php?f=30&t=16840 Myself and Jim (woodwizard) usually link up with the Toyota Surf guys when they have their annual Run To The Sun weekend, but this years has been cancelled due to health problems.
There are lanes around Plymouth area, some going right through to Torbay, some more (non challenging) ones around Newton Abbott.
The bad news is that I am shortly taking the Queens Shilling (well £2000 actually) and disposing

of my Fronty this week

after two happy years.
But if anyone wants to go to Balhu at any time, give me a shout and I'll lead you there (weekends only...its closed through the week) and I'll be your camera man