Yes, rail engineering work. No trains at all running - even the slow overground lines from Euston, or the trains from St Albans- this will be a major bus replacement operation.
Traditionally there is no bus replacement from Euston to Watford Junction, except on overnight services, due to alternative London bus routes existing, but that will take a very long time.
So the arrangement is to take Thameslink from St Pancras to Potters Bar, for a RR bus which runs to Watford Junction then follows the railway all the way north, calling all stations. It takes 2 1/2 hours from Potters Bar to Bletchley because the local roads don't follow the railway, so the bus keeps zigzagging to get to all the stations.
I think the most obvious route is the hourly National Express bus from London Victoria to Milton Keynes Coachway (journey time 90 minutes), then a 20 minute/8 mile taxi ride- no direct bus runs from the Coachway to Bletchley.
The other option (faster and more frequent) is Thameslink or East Midlands Trains from St Pancras to Bedford (40 minutes on a fast train), for a 40 minute RRB ride to Milton Keynes Central, then taxi or the Potters Bar bound RRB 1 stop south (13 minute ride) to Bletchley.
That is the ticketed route - taking 1 hour 49 minutes - 15 minutes past the hour from St Pancras 7 minute change at Bedford and 7 minute change at Milton Keynes Central, northbound, 9 minutes past the hour returning with a 17 minute change at Milton Keynes Central and a 14 minute change at Bedford.
