I took Rose to the vet this morning to have a tumour removed, but it turned out to be attached to the femoral artery and so they were unable to do anything about it. They called Mum, who called me at work and I agreed that it would be best for her to just not wake up again. The lump had already grown a lot larger in the few days between making the appointment and taking her in for surgery so it wouldn't have been kind to keep her alive without removing it.
Poor Lily is all alone now. It's not good for rats to be by themselves, but I don't think the time is right for me to get any more. Hopefully she'll be ok with just me for company.