She's definitely laying. She's sitting on the eggs of others too but she is our only hen that lays speckled eggs so it's easy to discern which are hers.
Thank you so much for the information and advice, I'll take it all into consideration :)
My 4 year old black rock hen has always been a little broody but normally it only lasts a few days, this time however it's been nearly three weeks and she's barely eating or drinking. She lays very rarely now (every few weeks/months) due to her age but since she's been broody she's actually...