My dear mental has gone and purchased a litre of 5w/30 fully synthetic to top up her Fiesta, now as far as I am aware current oil in the car is just an "ordinary" one.
Will there be any issues if I use this to top up instead of an "ordinary" one?
5W30 semi-synthetic in most Ford engines, but fully synthetic 5W30 is fine. I use GM's 5W30 fully-synth in my 1.8 Duratec (basically because I got a load of it for servicing my Poxhalls a few years ago and had a few cans left).