Here are a couple of links to another forum where it has been discussed in length which maybe helpful.
Re on airport car hire companies, if you book with economycarhire.com all the car hire companies they use are based in carpark 4 of the airport. Travelrent, drive4fun & Lagorent being the best recommended. make sure you read their fuel policy conditions ie: car returned with same petrol as when it was picked up as apposed to paying for a full tank of petrol when you pick the car up and bring it back empty, that's a moneymaker for them so avoid it at all costs.
Gerry, they don't seem to be doing very much at the moment, causing much confusion on how things work. I am not sure whether some of the top car hire companies have tags fitted in their cars, but the majority havn't so if you use the A22 your numberplate is read by the toll gantry and then goes onto the CTT (Post Office) payment system. The only problem is that the toll charge is not on the system for a least 48hrs after you have used it, that's ok if you are there for 7 days or more but if you are only having a long weekend then it becomes a problem. You can go to the post office, (or a payshop, no queues) give them your registration number and they will tell you what the payment is + admin charge and here goes another problem. Some people have gone to pay only to find someone elses previous usage (who havn't paid) on their reg number and from what I have been told you can only pay all the toll charges that are displayed on CTT system, so you are paying for someone elses usage. Another problem is is if you use the A22 for your journey back to the airport there is no way you can pay that charge because of the time lapse of it going on the system ie:48hrs. Apparently the carhire companies are keeping your credit card slip on return of the car so that if there are any charges you will be billed for them + an admin charge. Charges stay on the system for 5 days and after that if not paid a fine is imposed which is supposedly 10 times the toll charge. Not good at all in fact its a complete farce, hopefully it will improve, when is another thing. There was supposedly a new system coming into effect from 1 January2012 to 30th June2012 were carhire companies added a toll tax to your car hire charges whether you use the A22 or not but as yet I don't know whether this system is in working progress.