I have a 2.0 110 HDI GLX Estate. 138,000 miles young and currently adding over 3000 miles per month!

I quite fancy trying a slightly taller top gear ratio.
Can anyone help me with the following:
Does the 2.2 136 HDI run a taller top gear ratio? (the 110 runs 28mph/1000rpm)
Is the higher ratio achieved by...
a) taller final drive ratio?
or
b) different 5th gear pair?
or
c) Both?
I am trying to decide whether to swap the whole gearbox from a 2.2 into my 2.0 OR whether to try to swap the 5th gear set ( these seem to be accessible under the end cover with the 'box in situ in the car)
Thanks for your advice.
Peter