Paintseal’s Collins said application by qualified valeters was critical to Glasscoat’s five-year guarantee: “Unless the vehicle is thoroughly de-waxed, it won’t bond properly to the surface and for this reason we are very hot on unauthorised sources of supply like eBay.”

Collins said for a Glasscoat Gen-3 pack typically costing £50, an authorised dealer could expect to charge between £400 and £1,000.

Paintseal’s best-performing dealers reached 60-70% on sales across new and used, with one achieving 65% across the whole group.

In August, the company is due to start an online training programme, covering sales and valeting.

Permashine, based near Weston-super-Mare, says what differentiates the company is that it employs its own valeters to apply the treatment.

One of its customers is Wessex Garages, where a spokesman at its latest dealership, Hyundai in Newport, reported a take-up of just over 40% across new and used for treatment costing between £299 and £359.