In common with Mercedes-Benz and Jaguar, Northampton-based Linden Special Vehicles, the engineering firm that carries out the work, is very fond of superchargers. You can see why. Its 540i Sport, embellished with a Vortex blower, generates an M5-taunting 407bhp. Even more impressive is the torque: its 475lb ft makes even the ridiculously muscular RS6 suddenly looks a little weedy.

LSV's deliberately chose an auto 540i as the starting point for its demo car, because that's what most customers want, but you can have a manual if you so desire. Better still, LSV can import a right-hand-drive 540i to your spec, engineer the supercharger conversion and still undercut an official BMW dealer's standard 540i price. Alternatively you could buy a used 540i and then retrofit the conversion for ΂£8166 including VAT.

We've sampled LSV's work before, when we drove the supercharged HSV Holden GTS (issue 041), and know that it is beautifully executed and thumpingly effective. Unlike the HSV conversion, the 540i's supercharger isn't set-up for outright power. LSV knows that BMW owners aren't after a whooshing, whining monster and the Vortex supercharger runs at a modest 7psi. It's been developed to be both quiet in operation and progressive in its delivery.

Hushed and linear it may be, but when you're in the mood the LSV 540i is an utterly devastating performer. The car feels very keen from step-off but the power really starts to make itself felt at around 3000rpm. This is serious mid-range wallop - enough, subjectively at least, to ruin an M5 driver's day.

But the best is yet to come because just when you feel like the mid-range puff might be at the expense of top-end power, the supercharged Beemer hurtles towards the red line with what sounds like a hurricane blowing out of its stubby tailpipe. An M5 would be left choking on its fumes.

Getting a 540i to perform like this is an involved job and LSV doesn't simply strap on a supercharger and cheat the ECU into delivering what the technicians want by merely 'piggybacking' a new program. Instead they spent a lot of time and money on reprogramming the 5-series ECU and changing parameters for everything from the VANOS variable valve timing to gearshift programs, fuelling and throttle response.

The expense is justified by the homogeneous driving experience and absence of flat-spots. LSV also fits new injectors so that fuelling can keep up with airflow.

The supercharger installation is neat and is covered by a one-year warranty. LSV's experience suggests that the work doesn't invalidate the BMW warranty on items unaffected by the conversion - electrics, bodywork, etc - but it would be wise to get an official word on this before you went ahead with the work.

The autobox suits the supercharged Beemer's massively flexible delivery and as a relaxed executive express it's a convincing tool. If you're after M5 agility and ultimate driver involvement, though, the donor 540i chassis doesn't quite deliver. It manages to put its power down quite cleanly, unless you're being deliberately provocative, but there's never the tightly controlled feeling that makes the M5 such a rewarding steer. When the tail does kick loose the diff seems to be overwhelmed by all of that torque and reacts inconsistently. An M5 aces it for grip, involvement and handling balance by some margin.

The beauty of LSV's supercharger conversion is in its stealth and the way in which it delivers - the seamless charge from massive torque to dizzying top-end power is fantastic. LSV can now carry out similar work on virtually all BMWs, be they six or eight cylinder models. Apparently orders are streaming in for the supercharger conversion on the 330i, which gives 335bhp, M3-munching torque and all for less than UK list. Sounds like our sort of car.