Andy, just to give you an idea of what is available in surveyor's tripods. They will support some pretty heavy theodolites and other survey instruments. The mounting system is definitely not Arca Swiss. They use a large diameter hollow bolt on a swing arm so that an optical plumb can be used. I don't know this site other than they have a range of tripods. You may be able to find a surveyors supply shop in your vicinity and have a look.