And if ever a car deserved a performance uplift it's the Cooper S. It just doesn't quite live up to its fiery on-paper specification, thanks partly to a chunkier kerb weight than its rivals but more generally because the engine always feels a bit flat when you nail it, with none of the high revs fireworks you get from the majority of the S's competitors.

The Hartge conversion stokes up the S's fire to at least 200bhp; the great thing about German tuners is that they put their products through a rigorous 'TUV' certification process that guarantees the claims made for the mods as well as testing their durability. In fact, Birds, the UK Hartge agent, reckons that most of the Cooper S conversions already undertaken in Germany have come in at 212-215bhp on the dyno.

The bare bones of the conversion are a modified supercharger compressor housing giving better engine breathing, a different, high-speed belt-drive assembly and a revised engine management program. Hartge also offers an M3-style four-outlet exhaust, priced at around ΂£750, as fitted to the test car. It doesn't offer any more horsepower - the main restriction in the Mini system seems to be the standard cat and that stays put - but the sound's much improved.

The effect of the conversion on the Cooper S is little short of amazing. From the off the car feels transformed; the Eaton supercharger sounds even more frantic than before as it puffs up a real storm of air into the engine. But it's the way the power continues to swell way past the standard engine's 6000rpm peak that shocks. I wasn't expecting such a gulf in the power delivery characteristics between the two but the graph of the power curves tells the extent of the difference between the standard and Hartge versions.

The peak power figure may be up by around 47bhp, but by the time the engines reach 7000rpm there's a staggering 80bhp difference between the standard Cooper S and the Hartge-modified motor. Hartge is cagey about what exactly it's done to achieve this but it has to be the mods to the compressor housing that make the engine breath so much better; a speeded-up supercharger would give only a 10-20bhp increase, while the power curves would be a mirror image of each other.

It's the sort of power increase that would test any front-drive chassis but the Mini seems to cope well. Test conditions were cold and wet, so frantic wheelspin was easy to achieve in the first couple of gears as the Mini doesn't have a torque-biasing diff to help traction like the RS Focus; then again, you don't have the unwanted side effects you get with the Focus's Quaife diff. You can feel the steering tense up as the power flows to the front wheels but it never seems to suffer from the torque-steer you'd expect with so much power. But then the Mini has been developed from the start to have almost this much grunt.

The only problem the test car seemed to suffer from was a fair amount of engine shunt caused by slack engine mounts. Hartge reckons it was a fault on this particular car rather than a design fault, which seems plausible.

Independent testing in Germany has produced a 0-60mph time of 6.6sec and 0-100mph in 16.1; that latter time is a second off our RS Focus 0-100 time, but cross-country I know which car I'd place my money on to be the quicker.

The Mini 'Works' conversion has been slow coming to market because BMW is insisting it goes through the full BMW testing regime so that it has world type approval; right from the launch of the Works package, BMW wants to offer it in every market the Mini is currently sold in. So there's a Cooper S 'Works' pounding flat-out round a secret test track for 80,000km of endurance testing; obviously this takes precious time and money to complete.

Meanwhile, I reckon the ΂£1761 Hartge conversion is fantastic value for what it delivers. The ΂£4113 'Works' conversion will need to be very good even to match it, let alone beat it. And remember, you get 12 months' warranty on Hartge conversions while the standard Mini three-year warranty remains on every component not affected by the conversion, although the TLC goes out the window. A small price to pay for a conversion as good as this.