You will almost certainly get charged. I used to work for Flybe customer service way back when - it was openly stated among the staff that a major income stream was from the charges for checking baggage on the day from people who tried to bring too big a bag and it didn't fit for the overhead. It's by design. They used to make you stick your bag in a measuring thing before checking in and if it didn't fit "naturally" (i.e. no trying to cram it in there) you'd have to check it - and pay the on the day checked baggage fee.