Update.
I set 14 eggs between my cuckoo Polish hen and my wyandotte as they both went broody. I had 6 groninger meeuwen, 6 random bantams and 2 mixed breeds. Many weren't fertile or didn't develop far (shipped eggs), so I put the 5 developing ones under the polish hen and released the wyandotte because she had sat for long enough by then.
3 made it to hatch, and 1 died hatching so I had 2 lovely babies: a silkie polish mix and a groninger. Not a great result for 14 eggs but hey ho. Then only a few days old the groninger died. I let them out with their mama to free range for a bit, sat with then for an hour and all was fine, then went indoors for an hour and I think it got separated from the mama hen and another chicken killed it.
I didn't want the remaining chick to grow up alone as it might have real problems integrating when it's weaned, so I last night I went and bought some 4 day old polish chicks!
They are the same age as the mixed chick and I put them under the mama hen last night, and checked on them just before sunrise (my that was an early start). All seemed well. She has accepted 5 new babies, is keeping them warm and clucking for them to eat and follow her.
The only thing now is for the chicks to learn they have a mama! They don't know yet to go to her for all their needs. But they are learning fast. Well, some of them are anyway. Hopefully they'll all follow like little baby lambs.
I'll post some pics shortly.