Not totally sure of the vagaries associated with tank explosions and flash point / auto ignition temperatures except to say in my old industry I can remember 2 or 3 explosions associated with vented oil tanks where people were killed or seriously injured due to explosions. Non involved a direct flame and one was definitely due to external welding. Baring in mind, if I remember correctly Vg46 oil has a flash point of around 200 deg C, well above diesel. I personally would not weld a diesel tank without purging it with an inert gas, but if a yard is happy to do it (many do), then so be it. I would leave the boat with them and wander off, out the way!!!!!!!