I'll have to look into the Hall-Effect option, thanks :).
And yes, injectors run at a specific pressure which the fuel pump maintains, and injects fuel at a steady rate. The amount of fuel the engine receives depends upon how long the injector is pulsed for (the pulse width) basically.
I can't use the RPM method unfortunately as the fuel flow is determined by throttle position, RPM is directly linked to the road speed and the current gear (e.g. 3000RPM in one gear will be different fuel usage to 3000RPM in another gear, just as 3000RPM whilst accelerating will be more than 3000RPM while cruising).
As far as I can tell, on my engine (1.8l petrol Rover K-Series) there are four injectors, each one operating at 3.5bar pressure. The ecu sends a pulsed signal to them in order to allow fuel through. The pulse width and speed changes dependant on how much fuel is required.
What I want to do, is measure how many microseconds the injector is open in a given period of time. Multiply this by how much fuel the injector should pump per microsecond, and I know how much fuel has gone through! Very simple in theory 
All I want to know is whether I can do this without actually cutting and joining the injector signal wire with the one going to the arduino. I want to be able to simply take it to the car, clip the signal wire on to the injector, clip the earth wire onto body and away I go.
I hope this explains what I'm doing a little better 
thanks for the input guys.