Faulty "Faulty Light" detector

A while ago, a car parked behind mine decided to spontaneously combust, melting my rear bumper and the outsides of the light clusters, plus scorching the hatchback.

The light bulbs themselves weren't damaged and still worked, as did my aftermarket reversing sensor system, none of the wires were melted.

I got a garage to replace the bumper, hatchback and light plastics, but I'm having a problem with the "faulty light" detector as, whenever I turn on the lights, the bulb symbol on the instrument panel comes on.

All the bulbs work, I've taken them out, cleaned the contacts, tried disconnecting the reversing sensors etc, but I'm still getting the warning light.

I don't know what else I can do to sort this out, the MOT is coming up in a couple of months and, despite the functioning lights, I'm worried that the presence of the warning light will result in a fail.

Can anyone offer any suggestions as to what I can do next?