A third litter did arrive, so I have a final total of 18 baby mice and the two parents! The two boys from the 2nd litter have gone to their new home (I didn't think it was worth the risk of upsetting the established boy group to add them). The youngest litter is 3 weeks old so still in the girl group - but the boys will move out in the coming weeks.
I found a lovely set of dried grasses in the pet shop today - I only popped in for dog food and came out with treats for the mice and hammies too! All I can hear is happy munching.