one of the detecters on my Avensis has been pushed in causing the sensor to bleep constantly in reverse. Until I find time to take off the bumper is there any way to disable the sound.

Every sensor has separate cable going to main box (usually mounted somewhere in the boot, look for a separate cable going out of the reverse lights and follow it to the box). Most sensors are plugged into the box with RJ11 connectors or small jacks. Unplug damaged sensor from the box and voila.