@monty As with all PHEVs, if you’ve charged the car (even just once as in your case) the mpg as stated in the car will be completely inaccurate, as it takes no account of miles that you’ve driven on the charge, which you paid for but the fuel computer assumes was free! It’s an annoying aspect with PHEVs as you have to work out accurate mpg equivalent manually and it’s a lot lower than the computer says!
If you are going to drive it on petrol only from now on, you,d be wise to reset the fuel computer now (assuming that you’ve used up all the electric miles that you paid for). From that point on, your mpg readings will be accurate, as any miles that the car drives on electric will have been ‘paid for’ by the petrol engine.