So here is where I now am with it.
I've fitted the 9 channel receiver and connected all the servo leads to it. I haven't glued the tray into place yet as I still have to connect the rudder pull pull wires to the servo arm and check that it is balanced correctly. I think I'm going to have to add a little weight under the tray to achieve the correct balance.
The air brake (spoiler) servo was fitted and the servo arm extended. I then bent a piece of wire and attached it to the brake flap. So the servo arm goes up, pushing the air brake up and the wind resistance keeps it pressed against the arm. When the arm goes down, wind resistance and gravity do their thing and the arm then slides through the loop in the wire, so that when the brake is fully closed, the wire stops it from opening or being lifted by air pressure.
This was just a trial piece taped into place. I will fabricate another piece of wire and glass cloth/resin it into position.
I purchased KST servo mounts for ease of installation, but they also allow the servo to be easily removed should the need arise.
This is the aileron servo, and the flaps are much the same.
So I then connected the wing to the fuselage and tested that everything functioned as it should, and thankfully it does. I will set up all the mixes and throws once the model is complete.
This short video shows the functionality so far with flap mixed into aileron, aileron on its own, flap, and the air brake.