I have a PSI Powerbox on mine. Very easy to fit, Plug n Play. works well, can be removed from the car if you sell it. No need to interfere with the engine ECU.
Rover Ron did list one for the VP37 pump but just checking the web it seems to be discontinued. It was only about £100 so the cheapest decent one I have seen. Ebay ones tend to be just a resistor box to over fuel your engine so avoid this route.