Airbag SRS problem

[OllyH]

I'm new to the forum, hello all. Recently aquired a W639, '05 Viano. The SRS light is on because the front seats have been reconfigured - was previously a mobility vehicle with no front passenger seat, which is now installed. I hear different stories - Mercedes tried to sell me a new airbag control unit (£1000!), but I hear it can be managed in Star developer mode. Anyone seen this before? And/or know where I can get it fixed (near Essex, SE London, Kent). Many thanks.