Engineered in conjunction with Mercedes and VW, they work in tandem with the factory-installed ABS and ESP — Electronic Stability Programme — packages found on both vehicles. They come with hand controls and a fully automatic control integrated with the foot brake.

A retarder acts on the propshaft to slow a vehicle down, boosting safety, reducing wear and tear on the service brakes and resulting in a smoother ride. Fitted to ambulances for several years, retarders have regularly brought about a six- to ten-fold improvement in brake lining life says Telma.

The main drawback is price. The package designed for Sprinter and Crafter will set you back a hefty £2,650 fully installed and imposes an 80kg weight penalty.

That includes, however, an uprated alternator and battery, the cabling and the necessary electronic modules as well as the retarder itself.