the 450 had 6 gears where as the 451 has 5... which means the 450 is better on fuel than the 451.
The old petrol 700cc engine didn't meet the latest emission regs hence
the change to the larger 999cc. The 999cc is also a far stronger engine
and 'outlives' the 700cc easily.
The CDi engine at 800cc was unchanged as it was uprated to meet the
latest emission regs during it's lifecycle.
The other factor for the lower mpg of the 451 is that it is a bigger and
heavier car, but the benefits are that it has a far better ride and the
handling is also much better too.