Thanks to @ VAP for spotting this one-
On 3 December no LNER trains will be running out of Kings Cross to any destination. Passengers should use the Piccadilly Line to Cockfosters, then rail replacement coach to St Neots and train forward.
However this will also affect local Great Northern Services north of Potters Bar on the main line and to Cambridge- no trains will run and there will be NO replacement bus service (to Cambridge use services from Liverpool Street).
The following stations will have no service for the whole day: Arlesey, Ashwell and Morden, Baldock, Biggleswade, Brookmans Park, Foxton, Hatfield, Knebworth, Letchworth Garden City, Meldreth, Royston, Sandy, Shepreth, Watton-at-Stone, Welham Green and Welwyn North. There will be NO alternative travel options from these stations. No trains will run between:
Potters Bar and St Neots.
Potters Bar and Cambridge.
Hertford North and Stevenage.
A reduced train service will run between:
London Kings Cross and Potters Bar.
Moorgate and Potters Bar.
Moorgate and Hertford North.
St Neots and Peterborough.
Cambridge and Kings Lynn.
A limited rail replacement bus service will run between:
St Neots and Cockfosters (the northern end of the Piccadilly Line London Overground) operated by LNER- details here- this service was originally due to run to Bedford but has been changed due to industrial action on East Midland and Thameslink- details here- https://community.ricksteves.com/travel-forum/england/new-train-strikes-december
Stevenage and Luton Airport Parkway via Hitchin.- service to be confirmed owing to the industrial action.
Audley End and Cambridge North.
Welwyn Garden City and St Albans City.
However work to build the new Cambridge South station is also happening that day, so Liverpool Street trains are terminating at Audley End with bus replacements to Cambridge.
Thus the only all rail route to Cambridge is via (and changing at) Ipswich.