Top tip: look at the Youtube video from Espares: http://www.espares.co.uk/part/dishwashers/door-locks/indesit/di620uk/p/1529/805/0/553665/509247/dishwasher-door-interlock.html
The video shows how to get at the catch and how the catch looks when properly assembled. The small 1cm, 2mm thick bar sitting on the machine side of the catch pops out when the spring breaks. Found mine in the bottom of the door! The catch doesn't come out of a "built in" unit with screws, - it's glued to your kitchen door fronts, but nevertheless, with a bit of fiddling, you can get at the catch mechanism. Be sure hold the door down whilst you unhook all the cables inside- the door springs are likely to break them as it springs back after you've undone your last screw on the door cover!
RobO, November 2010