Inspired by kind Timmy Toad's comprehensive photos, and playing to my strengths, I scratch-built the missing parts to the C21 Tank last night.
I left the motor/ wiring for another day when I get more (any) powers!
The missing parts where all connected to the bridge winder and supports; a steering wheel and three rods. These allow the bridge to be 'steered' off the tank's turret and onto the ground. Timmy's photos made that clear to me for the very first time so thanks TT!
I raided the bits boxes and found these bits.