Not really my usual area - I'm more of a poltergeist and SHC man - but I've been dragged into a similar question by a friend in Dundee who spotted something over the Tay last weekend and wont stop going on about it.
What I'm trying to work out is whether the behaviour of these lights follows any pattern that distinguishes them from known atmospheric phenomena. With poltergeist cases you look for recurrence and intelligent response, and I've started wondering if the same framework applies here. Did the lights you saw respond to anything, change direction in relation to other stimuli, or just drift?
Also how long did the display last? Duration matters a lot when you're trying to rule out ball lightning or similar.