Oh come on, it's obvious there will only be one or a handful of the rare stickers that complete a set, it's not a secret, are people really that naive? It's just a bit of fun and obviously designed to pull in customers.
Moneysavingexpert publish the list of rare stickers every year, as do other websites.
I admit to going a couple of times more than I would, which is probably about 3 times instead of once while Monopoly is running, and I will have lunch there once this week, but after that, I probably won't go again for months, as I can take it or leave it, but they are useful for a quick cheapish hot lunch while out on the road.
DPs also been about the same number of times and we've saved the stickers and do have all the stations except the rare one, plus a few more, so far I've won a Now TV pass, which is good and a useless discount voucher for some online tat shop. Friend won two food items within the same visit, but obviously there's only going to be one Mayfair winner, so of course, almost no-one will know who it is.