Hi Nic,
I am also having the same trouble.
She is an FV610 and not up armoured.
Feels pretty strong on the flat, but start climbing a hill, and power seems to gradually diminish. I had an exhaust manifold leak, which we fixed with a new 2into1 and new gaskets. Cleaned the carb, new leads, new plugs, all to no avail.
I will keep trying, but not sure what to do next.