The position of the wings was driven by two features. One was indeed visibility for the gunners, as suggested, but the other was to bring the bombbay low to the ground to speed rearming. Quite a clever piece of design, if unorthodox.

Ivor Lumsden of the Isle of Man posted a photo of a wartime Heyford that convinced me as being in Yellow (undersurface and sides). I thought that this was on a Britmodeller thread, as mentioned above