Hi HG
It is the filler for the diesel tank
If you look at the top of this photo the tank is visible and it mounts under the rear cockpit floor
It is a square stainless (From memory but it could be alloy) tank that probably holds around 40 litres and has a sight tube on the front of it and a simple tap to shut off the supply
The small chrome firing above the tiller is the tank breather and the large round plastic part is a breather for the engine compartment
It has the filter on the engine and another below the engine that I will eventually replace with a filter/separator mounted up high so it is easy to get fuel up to bleed the thing after filter changes
If I was as keen as you I wouldn't mind cutting the cockpit floor out and putting a hatch panel in ti give me better access to the rear of the engine, gearbox and prop shaft but it will never happen
Regards Don