With full production (hopefully) less than a month off now, we should be getting some more casting news sooner rather than later. The preliminary publicity photo shoot featured only JB and Eve which means the remaining two "leads" will probably be minor players at the Institute.
My guess is we will soon get the announcement that a geeky scientist type and a saucy transvestite will be filling out the staff. Either that or the other spinoff rumour floating around might actually be referring to Torchwood. Yep, the last two missing members of the team are Sarah Jane and K9!