i know someone that was caught, a £500 fine on the spot for a first offence and tank random testing over the next year where he also has to show his diesel receipts to prove he's buying legitimately. second offence i believe and you forfeit the vehicle.
if your using it in a car your much less likely as it tends to be vans randomly stopped and checked.
at £1 a gallon from a building site near you its worth the risk.