Mersey Tunnel charges.

We returned from our trip today, and returned through the Mersey Tunnel in liverpool.We have been using the tunnel in our motorhome for the last four years and paid the toll fee of £1. 50 as we normally pay, but the barrier never opened one of the staff came out and told us we had to pay a £3 toll.We informed him we had always paid the same amount and he said we had just been lucky to get away with it.. Can anyone shed any light on this, we paid the extra money but we certainly were not trying to get away with anything