NB: I'm only speaking as a punter, not someone who's ever sold a bike. I'm also not a punter who's never really looked for a hybrid. Pinch of salt ready, in other words.
I poked around a bit just now by arranging Gumtree's offers by price and scrolling down until I reckoned your bike is shinier than the ones on sale. I'd say that was around the £120 mark.
It looks like standard practise is to put it in at more than you think it's worth, and then reposting once every two or three days at a tenner or so less; I think the ads are like forum posts in that once they're off the front page, they no longer exist for 85% of the audience.
I look most closely at ads where there are closeups of the mechs, bars and brakes. If I've seen that they're in good nick, the bike becomes radically more interesting.