Specsavers — overcharging at edmonton green

Last week I bought 3 sets of glasses from Specsavers. I was charged an extra £60 for slim lenses, and declined to pay extra for the other 2, and duely received thick lenses. It wasn't untill I went home and looked at their website, that the cost of slim glasses should have been £40, except, their was a special offer giving the slim lenses free. Also, Specsavers was giving 25% of for over 60s. This was not mentioned, or offered to me. The only conclusuion to be drawn is that Specsaver staff are deliberately ripping off their customers.
Insted of receiving 3 slim lenses glasses costing £100 less 25%, I was overcharged £160, and recieved 2 sets of unsightly glasses. Let's see if specksavers will rectify this.