I have worked on car engines since the 1950's and I'm very familiar with valve adjustments on many car engines.
I want to adjust the valve clearances on the above Briggs engine, but when I look under the rocker cover the only thing with a hex on it is the centre pivot of the rockers which needs to be tight or the valve will adjust themselves.
Are these valve clearances adjustable and if so where, or do I have to strip it down and lap the valves and grind the valve stem ends?

What your probably missing is that the valves are BOTH adjusted at the same time when the piston is 1/4" down on the power stroke.