Ok, so if we take the 1.4 GT, by using 70% of the combined MPG figure (which is based on my real world experience of my GTD vs the figures!) we would see 41mpg from that engine (manual gearbox)
Now the GTE has the same petrol engine, but also weighs more (batteries!), so I'd expect pure petrol power to have a lower real world MPG.
If you plug it in, you have an extra cost, as i said above.
If you charge it from the engine, your MPG will suffer, you will gain no overall economy (law of conservation of energy and all that) and indeed will actually loose some - unless its just charging from regenerative braking (which I think the car might use?)
So its really just down to lease payments (based on good old residuals apart from anything else), whether you drive in a congestion charging zone or if you can blag some free juice from your company...