I've managed to
download a revised timetable.
As far as I can make it out, on Saturday, buses replace northbound trains on the Cambridge line only (I'm in Hitchin, which is the junction). I'll be able to get my planned Peterborough train up and arrive about half an hour before the event
In the early hours of Sunday, buses start to replace trains on the Peterborough line.
Southbound, trains don't stop at intermediate stations between Biggleswade and Stevenage, with buses running between these. Going home, I can do some extra miles by hopping out at Biggleswade or, more likely, over-shoot to Stevenage and ride the five miles home.
Not too bad