Why am i using so much fuel recently?

I have a Renault T35 Master ex ambulance. it has been driving fine since i got it, but recently it has started using a huge amount of diesel. it is doing about 1 mile per litre!!! i cant find a fuel leak, but i am not sure where to look. the deisel is not being stolen, as each time i drive it it goes down even more. if there is a leak it is not affected bY standing, just when moving...HELP!!