FCC may be operating an hourly 12-car service from London to Peterborough as all the stations mentioned can handle 12 coaches. Arlesey, Biggleswade & Sandy can only take 8 coaches at present.
Also it suggests that the fast lines are closed in the Hitchin area so they cannot cross the junction - otherwise why would the Cambridge trains only operate north of Hitchin (they can start back from Platform 1 onto the Cambridge branch) ? Unless it is because the available paths through Hitchin/Stevenage area etc are very limited.
Sounds like services are also diverted via the Hertford Loop as FCC are replaced by buses from Welwyn GC & the bus replacement for the Hertford-Stevenage service frees up the loop for diverted services.
update:
it looks like there may be single line working or something as the journey planner on the EC site for saturday 12 Feb shows a number of trains close together then nothing for 90 minutes on a 2-hourly pattern.
looks like all EC are calling at Stevenage as is & GC services to Sunderland all call additionally at Stevenage, grantham, retford, doncaster.
Only 1 thorugh Hull Trains service seems to be showing -others start at Doncaster to Hull connecting out of GC service from Kings Cross.