My pereferd choice most of the time is to use easi dec conservatory roof access system
http://easi-dec.co.uk/store/conservatory-roof-access.html Bought it years ago as new from a fleebay private seller for £250.
I went for that make in the end because it has an adjustable stabaliser bar at the bottom of the ladder so can level the main section of the ladder if required which has proven does comes in very handy on jobs with uneven/sloping ground. Also the roof section comes with a kneeling tray for more comforatble working instead of resting your knees on bare rungs. Myself I don't use the roof section much as find it easier/quicker to use a wfp than having to keep taking the roof section off to move the ladder along.
My second prefered choice is to use a set of ladders with an akalad stabaliser fitted so can be used freestanding clear from the conservatory guttering.
Those easi-dec conservatory roof access systems can be transported on roof bars ok. However I found it easier and a lot less weight/hassle when transporting mine on the roof to take the bottom stabaliser bar, main ladder bar and roof section bar off and store them away in the van.