Water in the engine?
 

I need a bit of advice. I suspect a trainee petrol attendant accidently poured water into the engine (where oil is supposed to be poured in). Is there any way to check this without doing a complete oil drain\flush? I have driven the car since and it drives fine – engine is running smooth no white smoke, etc.

I'd at least look at the oil on th dipstick and the oil fill cap. If it looks milky then there is probably water in the oil.
But I wouldn't eve risk that. I'd just drain the oil and refill it to be safe.