Thanks so much for this link! There was an offer a couple of weeks back which has now vanished for free entry - I thought I was doomed to pay through the nose when ewe go back.
stitch - we used this offer a couple of weeks ago with no problems, in fact you don't need to queue up, just go straight to the ticket barrier and hand it in. We used 2, but I see no reason why unlimited numbers can't be used!
I think you can use as many as there are adults in your group iykwim. I found it on MSE and several posters had used it for family groups eg 2.4 kids, or 2a, 1teen and paying for the kids (it did say they'd tightened up on people using them for children)