Standard Focus' have RON adjustment so they can compensate. Not sure about the RS models but if you cannot afford to put V-Power or even high octane fuel in then you shouldn't be driving it I'm afriad you said it wasn't your daily so simply don't drive it if that's the case. Turbocharged cars especially do not like standard fuel if they are designed to run on high octane stuff. My focus even though is a 1.8 was custom mapped at MSD on V-Power so have no choice but to use it and 1.8 blacktops are ten a penny but still don't fancy knackering it by being cheap.