One very simple check you could make, is to inspect the bottom surface of the tappet which the decompressor bumps. Look for a local dent, or much more likely a groove right around, that surface, caused by the pin (tappets normally rotate while the engine is running).
If you have a sliding caliper (either vernier or electronic), please try to measure across the cam and pin, then across the cam immediately beside the pin. Alternatively you could lay a straight-edge across the cam and use the caliper to measure how far that pin sticks out above the cam.
In your pictures the mechanism looks oily but extremely clean Paul. If it was equally clean when you ran the engine, I don't think it was fouled up by gunk. Of course you should carefully feel the movement of the centrifugal weight, and of the pin itself, to verify that they operate smoothly.
The objective in all this is to get a measurement of how far the valve is being "bumped" by the decompressor during the compression stroke. If all else fails, you can reassemble the engine and watch the rocker arm for a "bump", both with zero tappet clearance and with the recommended tappet clearance. Then you can simply do a compression test with the tappet correctly adjusted, then repeat it with zero tappet clearance, and with an exaggerated tappet clearance. Because of the action of the decompressor, we would expect to find a much lower compression pressure with both zero tappet clearance and the recommended tappet clearance, compared with an exaggerated tappet clearance, which would defeat the operation of the decompressor.
