I decided that the unit was intended to be removable. The legs at the front are (I think) to keep it in position, stopping it sliding forward while it can't go back because of the driver seat will hold it. The seat isn't attached to the wall of the switch box so this makes sense.
Trouble was, it didn't seem to slip in and out very easily. I chamfered the front of the legs a bit and eventually mounted the window a little higher up so the curved panel didn't catch. It's not perfect, but I don't anticipate taking the thing in and out very often.
Not after I've fitted the motor anyway, something I really should have done first!