Which Is Best For The Environment, Bangernomics, Hybrid Cars Or A Conventional New Car?

The really short answer is simple.  Go for a car with good mileage per gallon and drive it sensibly.  Drive slower rather than faster, don't accelerate fast only to break again quickly.  Don't drive above the speed limit.  Keep the car well-maintained and try to keep it for many rather than few years.  And don't use it unless you really have to.  

Otherwise, pluses and drawbacks to each option:

Running an old banger until it needs real money spending on it: spews out more pollution, but saves on environmental damage from making a new car any earlier than necessary, or early disposal.

Few life-cycle studies have been done, but the ones that exist suggest that the most environmental damage (>50%) is done during the vehicle's operation (a vote against bangernomics).

Hybrid cars: Perform very well in terms of carbon emissions during operation.  BUT, more environmental damage in the manufacture, heavy so don't get such great mpg, may be tricky to maintain.  Disorganised owners will find themselves running off of the petrol engine much of the time.

Conventional new car: Gets the best mpg, spews out the least pollutants each mile (provided it's well serviced).

In spite of their higher particulate pollution, diesel cars, which get much better mileage than petrol engines, are generally rated as better environmentally than petrol cars.