Nerf N-Strike Elite Stockade - half price £14.99 and also included in 3 for 2, so buy 3 and they are £10 each.
These have been half price for the past 2 years. For some reason they don't appear when searching Argos for Nerf, you have to specifically search for "Nerf N-Strike Elite Stockade".
We've got one of these, very good for the price. Only problem is the motor noise, so very hard to sneak up on victims.