this mod sounds like nothing more than a resistor, which can be purchased at any electronics supply store at a cost of about $1.25 for a 5-pack.
i have installed this package - Red series APC/EPROM/injectors and along with the instruction sheets created and sent along with each package, courtesy of Swedish Dynamics, there was a mentioning of installing a resistor inline with the AMM signal wire. the instructions, if i recall correctly, were to use a 270-350ohm resistor. as it has been explained to me, though i do not thoroughly understand exactly why or how, this will trick the computer into believing it is getting more air than it is and thus 'compensating' by injecting more fuel. exactly what M-99.co.uk claims that their modification will do.
in my experience, this helped none and hurt a lot. i noticed no improvement in top end, a loss in the low end and idle would never go below 1800-1900 rpm. not only does this look and sound bad when sitting in traffic, it became a danger while attempting to downshift.
oh yes, and fuel consumption was altered just a bit... went from about 33mpg freeway to around 22mpg. in my honest opinion, this is a scam.
edit: i am very happy with the APC/EPROM/injector package that i purchased and installed, but i cannot recommend the so-called 'performance modification' that was in the instructions.