I was going to suggest a couple of drops of lavendar oil on a strip of fabric too. (And hold your nose when you open the lid of your bucket!)

Oooh, fabric softener isn't a good idea for your nappies, echidna. Are you using MCNs or terry flats? Terry flats are ok to soak, but it's not a good idea to soak your MCNs in anything (if its just water, no worries there.) Using fabric softener in the wash and using soaps for soaking tend to coat the fibres of your nappies and make them less absorbent.
I use vinegar in my washes but not every time. The vinegar wears out the elastic so it's not recommended, but I find it helps stop my nappies from smelling after they've been washed, so for me it is worth it.
I haven't used bicarb before so don't know if that would affect anything. I would think with the amount of water used in teh wash it \wouldn't make too much of a difference.
HTH!