Hi Daniel
I have a Cheshire tool and a laser tool and a barlow to use the barlowed laser technique. These appear to do a good job on my 8" GSO. I don't believe that the 12" GSO is significantly more difficult to collimate.
I'm happy to bring them round some time so you can experiment with them and see what would work best for you, before you spend money on one or more collimation tools.
PM me if you are interested.
Eric
