Customers who bought their vans before September 2012, when Mercedes began security etching the components at the point of entry, can bring their vans to workshops to have the job carried out free of charge. A windscreen sticker warns potential thieves that the part has been marked.

In addition to etching each catalytic converter with the vehicle's registration number, Hughes of Aylesbury is also spraying the unit with UV-tagged forensic paint.

Hughes' service manager Darren NMottingham warned that catalytic converters can cost up to £2000 to replace.