Thanks for all the responses so far.
I'm going to go with... the UV/blue sensitivity of the paper. (for now)
I have been aware of the limitations of paper to reds, but neglected to take the outdoor levels of blue into account.
My interior lighting is incandescent and warm-toned CFLs... probably not a lot of blue there.

John, I have been exposing at ISO 6, and have gotten consistent results indoors, so I'll stick with that.. (for now)

So the next step for me will be to get out and do some tests - I need a consistent factor to judge my exposures, or as Doremus suggests, one for daylight, overcast and open shade.

The series I am working on is a fun set of studies involving a $0.99 magnifying glass for a lens, and contact printing on Ilford Art300, developed in Caffenol. So far I have a number of really nice images. (I have lots of "proper" lenses in my collection, but am enjoying the extreme Galli-esque nature of the cheap lens)

Thanks again, folks.
Cheers,
Tom