I second the Cowm Quarry vote but note that only trials bikes have the right to roam the majority of the quarry. Moto-x less than 86cc are allowed in one area for the kids (actually pretty much anything for kids is allowed there incl small quads), and there's a bigger moto-x style track for bigger bikes and an enduro training area with rocks, logs, tyres etc.. There are days when a large enduro circuit is set up for more enduro/moto-x/trail style bikes and the quarry is closed to other traffic.. But you can't ever roam freely on a trail style bike, to my knowledge
Preston docks, the scrap of land opposite Renault/next to UU - be careful there. I've never seen police down that way, but I've seen plenty of the sort that don't look like they paid a good chunk of cash for their bikes if you know what I mean.. If your bike was 100% straight and legal then technically your only crime would be trespassing but I've often felt more like I'd more likely become a victim of crime than be judged a committer of it when observing people ride there
There are a few trails around the lancaster end of salter, getting up into cumbria/craven and more west, into the lakes.. Certainly wouldnt mind showing you round next time I'm out - drop me a PM here
I'd also recommend joining your local TRF meet - there's Lancs, Ribble Valley and Cumbria all within reasonable distance of salter, but you didn't actually say where roughly you're based..