[Grey V8 Pete]

Good job it happened at low speed! I would guess that the crack started along the edge of the weld in the heat affected zone. I would want to replace that flat gusset with a U-section piece, preferably wrapping it around the outer tube as far as possible on replacement components, as you need to suspect the other side of the car is also at risk of failure. So replacing both would be a good idea. I have experienced three failures over the years when accelerating from standstill. Two were half shafts (on a classic car) and one was a diff mount. There is a lot of stress on drive train and suspension components at this time. Peter.