Low engine oil pressure light

What was the condition of the oil (any foaming?) and oil level before you added 1L? Is the level in the engine coolant ok? 1L seems like a lot, was the oil level particularly low when you filled it? Had you recently done an oil change?

The oil was very low, and never done an oil change, only had the car for 6 months, and the garage filled up the oil when I bought it. I have only done about 5,000 miles with it.

If the low pressure light only came on momentarily or was only flickering before you added oil then you've hopefully avoided engine damage. If you drove for any length of time with the light on solid then you may well have toasted the engine. I'm surprised your car doesn't have a low oil warning before the oil pressure dummy light. My 2002/2005 VAG cars come up with a message on the screen when the oil is low but nowhere near critically low.

Six months is way too long to be driving without checking engine oil! You should be checking it monthly, along with the rest of your fluids. Did you just dump a litre in or did you actually check the dipstick and fill it to the correct level? As f91w said you need to fill it to the correct level and closely monitor it for further oil loss. Is there a creamy scum inside the oil cap? Is the coolant a bright colour or is it browny/blacky?

I had been checking it every month, and was going to do an oil change soon. Put a liter into in, because it was barely on the dipstick, got it back home and did my first oil change, got a friend who’s a mechanic to show me for the future. Less than a liter came out. And put about 4.5 liters in so now it’s running smooth. The light was just going on and off. When he checked the engine ECU, the engine had no error or faults which was strange.