For me, It'll be the Vegetable Casserole (page 127) followed by Baked Lemon Pudding (page 139), both requests from the family.
If you want to add to the occasion, either pop a 'Victorian Kitchen' DVD on (both during the cooking process and the consuming), or seek out the audio recordings of Ruth talking with her Basildon Park colleague, Audrey Hearn. They were recorded in 2010 and posted on the 'Audioboom' website by Basildon Park in 2012 here:
https://audioboom.com/playlists/3667-audrey-hearne-and-ruth-mott
Enjoy!
Happy Birthday, Ruth.