Hi Flossie! What a little champ arriving on her due date!
Anyway I too have a non sleeper after 3pm and have to say she has been like this since she was about 3 weeks old. Will she sleep in a pram or on you? I find that Izzy will sleep in her pram - (like she is right now) Ive been for a long walk and just leave her to sleep for as long as she likes. Also if I dont go for a walk and I feed her around 5ish she falls asleep on the boob so I just stay in my rocking chair and have cuddles with her while she sleeps there. Also if she isnt too grizzly maybe you can just have quiet play time during this time - reading stories etc before her bath??

Probably not a huge help sorry but I find that because Izzy sleeps so well at night that Im not overly concerned - it is weird though isnt it - she will sleep in her pram or in the car after 3 = just not in bed!
Good luck!