I've followed Micky's idea (thanks Micky) and it's an ideal place. I didn't need to relieve any of the plastic lip. Also fitted a relay to power the Centech unit with the ignition on, the relay fits nicely out of the way in the recess under the unit (and behind the battery). Any wires from battery or to accessories can also be routed through the recess, all nice and neat.
My Autocom is under the seat by rear light, I think the Centech is better placed near the battery as I guess most accessories (lights, horns etc) will probably be 'up front', but its probably a case of which is fitted first.
Cheers, Chris