If you go to the options page and turn on the Advanced Calculations option you can set the modulus below 1.0 mm
When doing this you have to specify some additional values to make the gear.
Hope this helps.
Thanks for your reply, that is very helpful.
Unfortunately there doesn't seem to be a similar trick for the bevelled gears...
Did you try Camnetics? http://www.camnetics.com/geartrax/index.htm
I think there is a 10 days trial version from this, it can be useful for you. Very simple to use!
We have used Camnetics Gear Trax. You really need it if you are actually designing custom gears.
Thank you everyone for your help.
I ended up trying two different plugins; eAssistant and Camnetics GearTrax. The former has a similar limitation on the module, but the latter does exactly what I need.