Thanks everyone for the replies.
I did have a quick look under the car, best as I could without getting it up on a ramp. I couldn't see anything obviously damaged or loose before coming on here to ask (not very mechanically-minded, myself I'm ashamed to say). The only thing that looked out of place was a tube that was hanging down a little between the centre of the car and the rear wheel, where it looks like a clip had broken which was holding it up. This wasn't down to the scraping noise that I heard though, I wouldn't imagine.
Yes, I still have the undertray. It is a standard spec Lusso, unmodified. If it continues, I will consider replacing the tray then, thanks for the tips. I presume that, if it was that, the scraping wouldn't cause any damage worth me worrying about?
It seemed to be scraping the post when moving forward or reversing, in the centre of the car, a little forwards of the driver's position.
It seems it didn't need much more clearance. I appear to have temporarily dealt with the problem by putting a bit more air in the tyres!