I was in Tesco (Reading) today and picked up a Smarties Egg (with tube) 158g (or thereabouts). It was marked up as £1 in the seasonal aisle. When it scanned, it did so at £2.50. I noticed on my way out of the shop. You get double the difference, so I got £3 back for my £2.50 outlay. The same ones were also marked up as £1.25 by the entrance, so if you might get £2.50 back and not £3.
Unfortunately I only bought one.
So you either get 50p back, or get it for free.