The official figures for the combined mpg for the petrol MHEV AWD is 40.4mpg VS 50.4mpg for Diesel equivalent. That’s about 25% more fuel efficient for diesel.
Today’s avg. prices for both fuels show diesel being 8% more expensive than petrol. This would suggest diesel is by far better for economy. But afaik, diesels are worse than petrol at low engine temperature, i.e. short journeys and fuel efficiency may be closer to the petrol before warm? I’d warn about a diesels DPF filter blocking up on short journeys, but if you already have a diesel, I guess your journeys haven’t caused this to happen.
Our short journeys (most of them) are up to 3 miles, plus it’s very hilly, so I completely ruled out diesel. As your short journeys are 10 miles, perhaps the diesel is still the better choice?
EDIT: Just seen that your short journeys are not city/stop-start driving, but A roads and motorway. I’m definitely leaning more to diesel being more frugal even though it’s about 8% more expensive. But probably not a massive difference between the 2.