IH is one of Britain’s premium van conversion specialists, with a reputation for outstanding build quality, innovative design and luxurious specification.

A few years ago, they branched out into building coachbuilt ’vans with the VW T5/AL-KO-based rear lounge J1000. Last year they followed this up with the superb J500, based on the mainstream Fiat Ducato chassis cab, and sporting eye-catching gold sidewalls. Launched at October 2008’s NEC, it was far and away the star of the show, wowing journalists and buyers alike.

The October 2009 NEC show sees the official public launch of the J500M, the Mercedes Sprinter-based version of the same low profile, fixed corner bed layout. This coincides perfectly with this summer’s new Mercedes engine range, which meet new Euro 5 emissions standards. There are two turbodiesel engines available – a 2.1-litre and a 3.0-litre, both offering increased torque figures at lower revs, and better fuel economy. The model we tested here was the 2.1-litre 129bhp 313 CDI.

Despite the like for like price increase of over £3000, IH expects to sell more J500s on Mercedes than on Fiat, no doubt because the three-point star is simply more prestigious and in keeping with IH boutique-style branding.

The Mercedes cabin is more elegant and upright than its Ducato rival. Chrome detailing to the grille is copied through to the wing mirrors by IH. The J500M opts for silver sidewalls and minimal, white graphics. Like the Fiat-based J500, it is a genuine low profile with an overcab mould that flows beautifully back to the caravan body and on to the rear, where neat light clusters and IH’s trademark boot panel for access to a generous storage locker complete the external looks.

On the road, there are many who prefer the Sprinter’s RWD to Fiat’s FWD. Why? When you’ve got a heavy caravan body loaded with camping gear, there are real benefits in having the driving wheels nearer to the ’van’s centre of gravity, than at the front. That way, the front wheels are used for steering alone, which results in fewer cases of scrambling wheels on hill starts, or spinning on a wet rally field.

The cabin is equipped with electric windows and mirrors, remote central locking and ABS. Mercedes options include passenger airbag (£465), automatic transmission (£1175), air conditioning (£995) and cruise control (£200).

The habitation quarter’s upmarket detailing – deep brown leather and the thickness of the cabinet work and worktops – tell you you’re in an IH-built motorhome.

IH offers the ability to specify exterior caravan body paintwork colours, and this philosophy extends to an unusually long list of options inside the ’van too. This is a crucial part of IH’s charm, the reason it can command high prices, and is a testament to Ian Hartley’s design nous: IH means hand-built, and what customers want, they’ll get (within reason). The layout will always remain the French-style corner bed, but beyond that it’s largely customisable.

The J500M is a hugely luxurious, dedicated two berth. Its fixed corner bed, side sofa with a cosseting wraparound backrest, and tub chair are sumptuously styled. With no seats for passenger travel, and no need to double as sleeping quarters, the lounge is the most comfortable we’ve ever enjoyed in a motorhome. The company logo is a detail worked into everything from the seating areas in the all-new showroom, to the several scatter cushions included in the ’van, the leather upholstery and the brushed-metal panels on the locker doors. High style livery or overkill? You decide. Personally, we love it.

Two free-standing tables dovetail neatly to make a full table, while either can also be sited in three different positions for cups of tea and the like.

The kitchen is another lavish feat of design and specification. Neatly finished with a darker wood veneer, it has deep stainless steel twin sinks and a number of storage drawers rather than cupboards. IH detailing extends to wooden cutlery moulds, with separate spaces for each item of cutlery and crockery. No plastic bin either: IH’s bin has its own wooden locker on the habitation door. Opposite the kitchen is the optional 180-litre fridge, with a double wardrobe sitting directly above it, inching out over the bed.

The bed is a classic French-style corner bed arrangement, with leather back roll and window pelmet against the rear wall.

The utilities are housed underneath the front end of the bed and include the combi boiler, fresh water tank (if specified on-board), water pump, battery and charging system, with some limited storage space, too. The larger storage space is accessed through the rear boot, where a vinyl floor makes for easy cleaning, and where the toilet cassette is also serviced.

The bathroom is the icing on the cake, though. It has a blue lit cabinet against the rear wall, and a rain-type shower head. Our test ’van came with upgraded LED lighting and a stainless steel shower curtain ring. The stainless steel basin and soap dispenser are touches you just don’t get elsewhere.