>Does paying the 18.50GBP for each bicycle make it a totally separate unit, and are then >allowed to pay a fee for the other 20KG bag we want to carry?

I have no experience with easyjet, but this ist the way it works normaly.

>What if the bike box weighed 25kg to 29kg??

At Easyjet they write, that you'll get 12kg for your sportequipment, so only 3kg will remain for e.g. your Ortliebs :-(