I think they've burnt their bridges with Derek. He went full time, but then he wasn't seen again after the redundancies last year - so I doubt he'll be in a rush to go back.
Their freelancer of choice before the redundancies was gobby Clair Cummins - but they don't seem to be using freelancers at the moment either, so my gut instinct, as she's been covering a lot of Gems TV recently is that Vicky Gordon will become Hattie's replacement.
I suspect when Jess Foley goes on maternity leave, they'll just get Lindsey Carr to cover more, as she does now, with others chipping in too to bridge the gaps too - perhaps people like Jim or Cheralene.
As an outside bet, they may take Millie on again yet again - but after losing her job three times, she might just be happy freelancing now and again instead.