Thanks to everyone for the replies, my last trip was in oct 2010 and there was no mention anywhere of changing the booths.
jills link gives the information about the charges from Dollar. I am thinking that buying a sunpass transponder may be a wise move long term. also, if I see a cheap sat-nav on this trip then that's a possibility as well.
I do think Dollar may try to sell/push this when we get there, if so , i'll post here.
I'm off to bed soon as I've got a 7am start, driving up to Manchester in the morning, for the 12.35 flight.
again thank you to all, as the replies have eased my mind now.