In reply to CurlyStevo:
The downside of Rosyth is that it sees substantially more climbing traffic than Blackford. Though if you go midweek daytime it should be fairly quiet (so long as you avoid school holidays!)
I think the longest lines at Rosyth are in the Heathy/Cathy area, so about 15m according to the UKC routes database. That's VS territory so steep enough (well, it feels steep enough when I'm climbing there, anyway!) The Inn Pin ab is 20m according to the book. Not sure exactly how long the longest Ratho routes are but I'd guess longer than 15m.
Come to think, a pal and I ran an abseil session for a group of non-climber friends a few years ago, using Cambusbarron West Quarry. IIRC the line we chose was a bit to the left of the tumbledown buttress in the middle of the crag. I think we used a combination of a fence and a tree as anchors for the ab and safety ropes. According to the UKC climbs database the routes here are ~10m but it felt like more than that after paying out the safety rope and hauling it back up again a dozen or so times! Might be another option, although in the Stirling area than the Lothians. (Not that there's anything wrong with Stirling - apart from the confusing road network. But then I'm not a local; maybe it makes sense when you live there.)