Tow bar electrical problems on Mercedes E Class

I had a detachable Westfalia fitted to my Mercedes E Class estate.I was very impressed with the work and all went well.

 

Around two months ago I started getting electrical problems with my car.The interior lights flashed at will, the headlights did the same and I got worrying warning messages on the dash that the seat belt restraint system was not operating correctly. This warning message required me to "Consult your dealer immediately".  I therefore took the car into a Mercedes specialist garage in order for them to have a look. His conclusion was that the problem  related to a fault with the tow bar electric connection. He therefore disconnected this and the car's electrical problems are solved. No more disco lights! Marvelleous!

 

Alas I was charged £270 for the investigative work which to my mind was ultimately caused by the defective kit / fitting.

 

I have gone back to the tow bar compnay and await thier reply. Has anyone else had a similar experience?