Posted: 01/08/2016 at 08:16
I can't imagine any lightweight trimmer being up to the job of cutting that amount of hedge so it really is a petrol trimmer job, as you suggested yourself. I would advise going along to a specialist garden machinery supplier so you can get their advice and actually handle the various options.
Cutting the high side is going to be problematic whichever route you take. Long reach trimmers are unwieldy and get very heavy to use, very quickly. You really need a movable platform of some kind, whether that be a well build stepladder or a builder type platform. I have a 13 in 1 ladder which converts to a work platform standing roughly 3' high. That would probably be high enough to access the high side of your hedge, assuming flat ground at that side.
The ladder is similar to this http://www.carvers.co.uk/products/MAXIM-13-WAY-MULTIFOLD-LADDER/P144281