but doesn't petrol have to work harder than diesel to achieve the same level of work? Its my understanding that diesel is more energy dense than petrol...
Not sure how you define "work harder". If you have two engines of same displacement with same gearing in otherwise identical vans, they would have to produce the same power at the same RPM to motor the van down the road at the same speed.
Same horsepower and same RPM means same operating torque, so if both engines were same size that suggest same average cylinder pressure. In other words two identical Transits with same 3.73 gearing as an example, cruising at 70 MPH require the same average cylinder pressure whether 2.7 liter engine is diesel or gas (unless of course one shifted to different transmission gear).
Diesel and gasoline engines exhibit different cylinder pressure profiles but I wouldn't necessarily assume diesel has the advantage. In the past diesels we all loved had to be built much heavier because combustion was abrupt -- hence that diesel knock sound. Newer engines have sophisticated fuel injection that minimize that pressure surge, but for the most part it seems to me that Diesel engines are still built heavier.
So if a brand new gasoline engine was designed for diesel-like operation, what would keep engineers from making it beefier to last under higher stresses? I've never been a fan of taking a car engine and mounting a turbo on it to increase power and torque and calling it good. I know they get all kinds of upgrades but it's not the same as a new design. And personally I think that may be why the 2.7L EcoBoost doesn't have an aluminum block. Starting from scratch like if it were a PowerStroke diesel the engineers can beef it up as needed.
Not suggesting it won't be crap because it may be, but I wouldn't personally rule it out based on small size alone.