We don't get the oil driven carburetor heaters in the US so take my advice with a bit of caution.
The oil for the carb heaters flows from the righthand side of the cylinder head, through oil supply tubes to each carburetor and then back into the engine. On US model bikes, without the carburetor heaters, this oil passage is plugged with an M6 bolt. If you want to plug this, get an M6 x 1.0 x 15mm (approximately) bolt and a small copper washer and thread the bolt in. This should resolve your leak without affecting the engines lubrication system.
