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My son purchased a Mazda Demio 2005 six months ago and has had to put a litre of oil into it on two occasions in that time.

Would you consider that normal usage or as I suspect that there is an oil leak which should be investigated?

It depends on the mileage he has travelled in that time, the type of use and how the car is driven. However, engines will use some oil and 1 litre every 3 months may not be excessive.
Engines that are driven on the open road at high speed will generally use more oil than those driven more gently at lower speeds. As an example, many car manufacturers state that 1L/1000km is acceptable, however for New Zealand conditions this would be excessive, but if driving at high speed on Germany's Autobahns every day it may be OK.
It's unlikely that the oil is leaking in this quantity as I think you'd notice it on the ground, so it's more likely that the engine is burning it.
Perhaps it would be worth mentioning it to your mechanic next time the car is serviced, just to get them to check it (for leaks too).