As I understand it, the FCC services which were working through from Peterborough to London were operating as 12-cars, so could only call at those stations with sufficiently long platforms - i.e. Peterborough, Huntingdon, St Neots, (Hitchin?), Stevenage & Kings Cross.
one 8-car service per hour was running through calling at the other stations(e.g. Biggleswade & Arlesey).
a half-hourly 8-coach service was also running Peterborough-Biggleswade where buses were substituting to Stevenage.
Likewise, all Cambridge services were terminating at Letchworth with buses running to Stevenage (though most buses seemed to run via Hitchin - on the coach I was on only one person got off there so only one more could get on at Hitchin so it would have been better for most buses to go direct to Stevenage with a separate bus running via Hitchin).
At Stevenge, there was a service every 20 minutes calling Knebworth, welwyn north, Welwyn Gc, Finsbury Park & Kings cross. (The FCC website did not mention this service pattern but it was evident from the screens as soon as I arrived at Stevenage.) These were starting from platform 2. (Northbound services had terminated in platform 3 & then reversed on the trailing crossover north of the station)
There were regular announcements which proved to be accurate.
The trains coming in from Peterborough & east coast services were using the slow line through Stevenage to Woolmer Green.
I was only delayed by 35 minutes (leaving Letchworth on the bus at 7:15), though I understand that people leaving later than me were more heavily delayed by an additional hour because Hitchin & Stevenage had become clogged with traffic (combination of no trains, school traffic & roadworks & the snow had got heavier by then), so they took much longer to reach Stevenage station on the bus.
The Station staff at letchworth directing people on to the buses also were giving out accurate information, e.g. saying that there were regular services to Welwyn from Stevenage. Sometimes during disruption FCC seem to ignore Knebworth & Welwyn North & run everything fast to Finsbury park - but this time they ran a decent service. I think welwyn north & Kebworth had a better service than usual yesterday.