Hi Mick,
Something like this is the easiest and cheapest way to go. I have had one for about 7 years, but did need to replace the transducer once. The transducer is installed in the main fuel line and has a paddle wheel which measures your consumption.
Regards,
Dave.
PS, originally I had it fitted to a 130hp yamaha.
PPS, this site won't allow the link I wanted to use, but it was a Northstar fuel flow meter (model F 210). Available all around the place, including J. &H Products.