Only thing I was disappointed in is that Top Gear do occasionlly make serious points, especially on environmental issues, yet they failed to do so on the diesel electric hybrid, which is an area that's actually worth continuing with.
Electric motors are pretty damn efficient regardless of speed, petrol and diesel engines reach peak efficiency at a certain speed and at other speeds are less efficient. The point of diesel electric is that the generator can, in theory, run at peak efficiency, then when the batteries are full you just switch it off until the batteries are getting low again. You never have to run the diesel engine at a non-optimal speed.
They used it on trains as it saved fuel over a straight diesel engine. On a car you could combine it with regenerative braking and all the other stuff Toyota have been doing and get a genuinely efficient car unlike the petrol hyrbids that in reality only get a few more MPG than their petrol-only counterparts.