Bit of a mystery car, this. Obviously a Rover 75, but, as it passes by, you can see some people doing a puzzled double-take, as if there's something curious about it that they haven't quite identified. Then, although it's good to see the Vanden Plas name back in circulation, if Rover is at Longbridge how come this model comes from Walsall?Why are there no official performance, economy or CO2 emissions figures? Did I hear somebody mention a six-door variant? And what's all this stuff about the factory being equipped with a firing range where they blast off armour-piercing ammunition?Yes, it's an intriguing prospect all round. Introduced at this year's Geneva Show and launched on the UK market in June, the 75 Vanden Plas is a special-order but very reasonably priced way into the limousine market. It may not have a glass partition behind the driver's seat, but it's a beautifully crafted long-wheelbase conversion absolutely not to be spoken of in the same breath as the expression "stretch limo".While some cruder extended-length conversions need vinyl roof coverings to hide the hacksaw marks and welds, what old-established specialist coachbuilder S. MacNeillie & Son of Walsall produces is an impeccably finished V6-engined 75 Connoisseur automatic, fully approved by Rover, with 200mm or almost 8" of extra wheelbase and body length.You might think the sky would be the limit when it comes to add-on cost, but the Vanden Plas attracts a premium of just £4425, and it's sold at under £28,000. That seems to me to be excellent value, and it's worth bearing in mind that Rover and MacNeillie worked very closely on the design, development, construction and engineering of the project. It's nothing like a Friday afternoon "Yeah, well, not a bad idea. Let's give it a whirl."Going back to the questions at the top of the page, for an observer who hasn't noticed the extended wheelbase it's the unusually wide rear doors which seem out of kilter. The Vanden Plas starts as a standard 75 Connoisseur off the Longbridge production line and is then sent to MacNeillie's works at Walsall.(Incidentally, while Vanden Plas was originally a Belgian coachbuilder, Austin bought its British branch in the late 1940s, which is why the name can be used by Rover today. Vanden Plas, of course, was a marque name in its own right for several years, when its catalogue included that rare machine the Princess R, fitted with a 3.9-litre Rolls-Royce military vehicle engine.)While the standard car has audited performance, economy and CO2 emissions figures, there's no requirement to provide these for an after-production conversion. The six-door version is mainly for the funeral trade, which MacNeillie also supplies with hearses. It does minibuses too, for seated and wheelchair passengers.What it does to the 75, as delivered from Longbridge, is - well, nobody likes to say cut the body in half, rather that it's "separated", and then has the 200mm extension inserted on a jig. New body parts include a full-length roof, different rear doors and windows, longer sill finishers and so on.Once MacNeillie has finished, there's no sign from the outside that it's ever worked on the car at all, except that, according to my chauffeur (more about that in a moment) it's just possible, if you know what you're looking for, to see some evidence in the brightwork along the waistline.Inside, the Vanden Plas is a revelation. The leather-trimmed 75 Connoisseur has a polished interior presentation in any case, and the far better rear passenger space completely transforms it. "Limousine-like accommodation" is exactly the phrase.The new roof panel provided by the MacNeillie conversion has additional reading lights, plus one of those brilliant ideas which just "light up" a car. I've always admired the retro instrumentation on the 75, and there on the rear roof panel is a duplicate of the clock on the fascia. Lovely little touch.When you get out on the open road, the 2.5-litre KV6 engine and standard JATCO five-speed automatic transmission don't find the extra weight of the Vanden Plas bodywork much of a problem. This is no sports saloon, of course, but it was much more "wieldy" than I'd been expecting on some country B-roads, still showing a fine ride quality, and it wafts easily along motorways.I hardly noticed the extra inches in the wheelbase, although I did try to keep farther out and turn in less vigorously than usual on sharp left-hand bends. The one thing you have to keep particularly in mind in town driving is to avoid getting too close to the kerb when turning left into side roads or at crossroads and T-junctions. Swinging round too soon could cause alarm and despondency among pedestrians waiting on the edge of the pavement.And the chauffeur? Well, Rover had the most acceptable idea of sending me out with one of its own staff, who after a while ushered me into the back seat and took the wheel himself. That was certainly a splendid way to continue motoring, but the Vanden Plas is just as convenient for an owner-driver whose family or colleagues would appreciate more rear seat space than the standard 75 offers.Pearlescent finish on the Royal Blue paintwork seemed a reasonable £450 extra, and I think I'd certainly pay the additional £225 for a wood-rimmed steering wheel. After all, the Vanden Plas is competitively enough priced to allow for a few extravagances. Engine 2497cc, 6 cylinders Power 174bhp Transmission 5-speed automatic Fuel/CO2 See text Acceleration See text Top speed See text Price £27,995 Details correct at publication date