“If you want to turn up outside the best restaurant in Saint-Tropez and have people say, ‘Wow, what is that boat?’, then you arrive in a Sunseeker,” says Sean Robertson, the company’s sales director. Face tanned from years of salt air and Florida sun, Tudor Black Bay on the wrist, Colgate smile at the ready, he’s a man who knows luxury boats and how to sell them. Thirty-three years and counting. “Sunseeker has been my life, really.” He’s had stints in Mallorca, Mexico, Japan, America, and currently — in a nondescript, strip-lit office set just off a roundabout with signposts towards Dorchester, Blandford and Ringwood, the wind rattling impatiently against double glazing — Poole, Dorset. An unlikely hub perhaps for one of Britain’s most successful independent luxury brands, celebrating 50…