My suspicion here is that when they replaced the door, something was out of alignment. The garage who replaced the door tweaked something which stopped the sensor activating for a while and it's now gone out of alignment again.
I think you have 2 choices;
1. Get the garage to realign the door. They did it before and they can do it again. Any body repair work should be remedied within a 2 year period if it is faulty. You should try to get them to do it FOC argue if they try to charge you. or;
2. Turn off the sensors in the settings menu. With an older car, I'm not sure it is worthwhile spending money to fix these things. e.g. I have a leaky tyre valve and it's easier and cheaper for me to just replace it with a standard one and turn off the sensor.
Good luck with whatever you do. Anyone else have any ideas?
B Max Titanium X 1.0 125 bhp in Magnetic with built in SatNav and rear sensors.