Well.. lifted the cylinder head and discovered the cause of the rattling/tinkling.
No4 cylinder where the noise began - the spark plug tip was hammered and smashed. It wasn't the broken plug tip that was the cause of the problem, the plug was the result..
There was a small piece of alloy in each bore. The alloy had been well and truly hammered by the pistons of course. In one of the photos you can see pitting/hammering on No4 piston.
There's no damage to any of the bores, or seemingly to the valves and seats. The removed cylinder head was turned upside down and the valves submerged with wd40. The wd40 stayed there, indicating that the seats were holding their seal.
It begs the question where did the alloy come from? The injection unit had been removed to fit a new gasket and I can quite understand accidentally dropping something down through the inlet manifold and it finding its way into a bore. But, I'm 100% positive that nothing was dropped.
The other point would be that bits of alloy were in all four bores. It goes through my mind that bits of alloy swarf were already lying somewhere in the injection unit housing/inlet manifold area, and that the flooding of petrol washed them down. I really, really can't see bits of alloy finding their way down through valve stems. Besides, the inside of the rocker cover was inspected - all seems in order. There's nothing seemingly amiss with the fuel injection housing either.
Water under the bridge now... in the next few days I'll re-inspect everything and then get round to refitting the head. The photos of the 4 bits of alloy, plug, head ..