A friend who worked for Toyota told me that Brinks make the towbars and the wiring could be obtained from [url="http://www.rightconnections.co.uk/links.htm"]http://www.rightconnections.co.uk/links.htm[/url], as they source their kits from the same place as Toyota. The wiring kits have dedicated sockets, allowing easy connection, without the damage to the wiring that you get with Scotch Locks.
Whole lot came to £203, complete with a removalable hitch. Towbar took 25 minutes to fit and the wiring about an hour and a half, but I didn't remove the corner bumpers.
Have now fitted the rear step and is a perfect fit onto the towbar.