Do you really want to do this? The kit will expect 125mm and by cutting that
down to 100mm you will be severely restricting the airflow. I have just
taken delivery of a new cooker hood with 125mm outlet and while I agree that
it's more difficult to get 125mm ducting stuff than 100mm - especially with
the ducting in aluminium which I wanted because it's going to be visible -
it is possible. Try:

http://www.domusventilation.com/

NB I found it astonishingly difficult to get hold of their stuff even when I
had identified the exact part numbers from the catalogue - nearly all the
stockists suggested by the manufacturer said they weren't (stockists that
is) or had a few bits only in stock and couldn't/wouldn't order the less
common stuff for me. Domus did eventually point me to Starkey Systems on
01905 611041 or http://www.centralvacuums.co.uk and after a couple of
failures the stuff arrived mail order.