Unfortunately the only way I know to turn this off is to take it to a LR dealer who can plug it into their computer to reset the light, some LR specialists can also reset the light. Either way, I would expect to pay no more than ½ an hour of labour for this service.
I would also expect the tradesman who painted the door to pay for the labour. I know of no car with SRS in existence that allows you to disconnect and reconnect the air bags without disconnecting the battery first and not throwing a code, it's part of the standard self diagnosis system (you couldnt safely rely on a system that has an intermittent loose connection). Quite simply, he should have known better.