I have been over a few times with no vehicle due to crossing problems or last minute booking due to work. I found the bus services to be pretty good with very friendly and helpful drivers, they run fairly late as well so can be handy for a few shandy's after the racing, they use to offer a discount scheme which I think you could purchase say seven days unlimited travel which if you were to use the bus a few times a day would provide a good saving, we got ours from the sea terminal information desk. As regards to you're marshaling maybe you would be able to have a lift or something off someone who has a car who's also marshaling, it's never as good as having some form of transportation of you're own but I think the busses do provide a good service and run pretty frequently as long as you're staying within walking distance of the bus route. I had to bailout this year have a good safe one and fingers crossed for the current bun to shine on the rock have fun.