Don't buy any more oil from the garage Milly! £15 for 1 litre is a terrible rip-off. My brother stopped on the motorway for oil once and they tried to charge him £45 for a gallon (or 5 litres)! Go to any motor parts factor (supplier) and they'll charge you fairly.
If the oil light has gone out you have just the bare minimum of oil in the engine. Buy a 5 litre can of oil then slowly top the oil up until it is level with the top (full) of the two marks on the dipstick. Get your father to show you how if you need to and then you'll always know. You car should take about another 3 - 4 litres at a guess. It doesn't matter too much if the level is slightly either side of the full mark but fill it slowly and try to get it exact.
Make sure you don't overfill the engine. My first wife once told me to my surprise that she would top the car up with oil, then got all funny when I asked her if she knew how. OK, OK, I thought. I then got a call from the petrol station saying the car wouldn't start and had a flat battery. When I got there with jump leads the engine was only turning over very slowly. I eventually found she'd continued pouring oil in until it was up to the top of the filler cap!