Hooray, operation 'Stop Phili being broody' was a triumph! Since Saturday afternoon she has been back into normal chook behaviour.
I had been tipping Phili out of her nesting box each morning and afternoon, but on Saturday I waited until Butter had laid her egg and then removed both nesting boxes. On Sunday morning I put one nesting box back into their house for Butter and then removed it when she had laid her egg.
Thankfully Phili didn't try settling down in the dirt, or I wasn't sure what my next option would be!
The only sad note is that her time away from normal chook life has made her forget how fabulous bugs and grubs are to eat. I don't think Butter minds though as she now gets to eat them all!