Just received this from TfL:

From 00:45 on Saturday 10 until Wednesday 14 February, planned works will affect Piccadilly line services:
• From 00:45 until 05:00 on Saturday 10 February, there will be no service between Hammersmith and Heathrow Airport (all terminals), meaning there will be no Night Tube west of Hammersmith
• From 05:00 on Saturday 10 until end of service on Wednesday 14 February, there will be no service between Acton Town and Heathrow Airport (all terminals)

All part of the Piccadilly line upgrade apparently.

EDIT: See update in post 10: https://www.flyertalk.com/forum/35984395-post10.html