But the Oil has not been changed has it, only topped up.

Much more likely not enough oil in at the Service and the light came on and then you never put enough oil in at that point, 

just you put some in and the light goes out because the bonnet was open, even if you had not put oil in, 

but you did put in oil, level still too low and again the light came on.

 

Now at correct oil level, all well.

@Robertino did you happen to check the oil level after the last service before it put the warning light on? If not, dealer might not have correctly filled it in the first place so it was already low. Just a thought!

The service invoice should have the type and quantity of oil used at the service.