It could be anywhere from 25:1 (oil/gas) (I'm in the U.S.) to 50:1. If you don't have the owner's manual, try going on line for the manufacturer's recommendation.
Once you retrieve the correct ratio, here's a calculator to determine how much oil for a given amount of gas (your petrol): http://www.csgnetwork.com/marineoilfuelcalc.ht ml
Best of luck!