The biggest problem was the general "floppiness" of the assembly. The 4mm ply has no great torsional stiffness, and the fin has quite a moment arm against it. When trying to man-handle the fuselage, this causes all sorts of problems, and I could tell straight away that things were not going to be built straight or level (not exactly uncommon for one of my builds!).
So, I built a building jig :
Now onto the wings :
Razor saw included for scale. This is the underside of the port wing, and the flap canoes are laid out in their respective positions. The thick black lines are the engine pylon mounting datums.
Happy with all that, I rolled the wing over to start marking up face cuts for the flaps and ailerons.