You're going to need to be pretty convincing to justify that length of time in a B & M. Did you spend a lot, have you still got the receipt? B & M is your best bet to get it cancelled, provided you really didn't spend that length of time in store.
Not sure what the 29 days comment refers to above as I think that is correct. The act allows the PPC to transfer liability to the keeper after 28 days, I.e. 29 days.