Morning @kezo I am ploughing through the 700 page manual 🥵 Perhaps it’s just my copy, but whilst the text is clear the pictures are extremely poor resolution, which doesn’t help.
Anyhow, it’s  just dawned on me why our figures are different and your figure is probably the more accurate. I had used 70 miles of the 320 as EV only and 250 on petrol. This, of course, overlooked the fact that the battery never goes below 15%, so you still get a small element of EV driving (low speeds, coasting etc) over and above the 70 miles that the 15-100% provides.
That explains how I got to £38.25 petrol in my calculations when your actual cost when you refuelled was £34.09. The car saved you that difference through the 15% that the battery holds in reserve.
This has been a really interesting exercise, because I now finally realise how I can work out the ‘true’ mpg equivalent of my PHEV;
1) Work out the money you have spent (£2.60 per home charge plus cost of petrol – as you did), in this case £39.29
2) Divide this by the total miles driven (again, as you did) to give a total pence per mile cost, in this case 11.2p
3) Using the cost of fuel (£1.375 per litre in your case – that’s cheap BTW, it’s £1.409 here) work out the cost per gallon (£1.375 x 4.546 = £6.25 per gallon)
4) Divide the cost per gallon by the pence per mile to get the ‘true’ mpg equivalent, had all the expenditure (£39.29 in this case) been on petrol only. In your example that would be 55.8 mpg.
Does that sound right to you? Where I am still in doubt is that 55.8mpg is better than the 53 mpg shown in the car, when I’d expect it to be lower due to the electric usage 🤔 I can only assume that’s because the 53mpg only related to 320 out of the 349 miles and the other 29 miles might have been on electric (?) so had the computer not reset after the first 29 miles the mpg stated in the car might have been showing a lot more than 53 mpg. Does that make sense? I’m determined to get to the bottom of working out the ‘true’ mpg equivalent to know if I made the right decision getting a PHEV and I think the above formula sounds right but, then again, I did last time 🤣