Just to add to this now, Stagecoach have confirmed the coaches will be returning BUT only on the section between Oxford-Bedford (still the X5). It will be split at Bedford from 30th August and the eastern section operated by double deckers - this appears to be a permanent change as the new 905 (Bedford-Cambridge) will serve North Cambridge (for the CPCA it seems). Understandable for such a long route to split it for punctuality, but I wonder how it will impact what seems to be a predominantly leisure market on this corridor, who may be averse to changing buses. Obviously very few passengers going the whole way, but I would think quite a few crossing Bedford. Either way, the days of the Oxford-Cambridge X5 are numbered.
The eastern part was very popular with student and commuters between St Neots and Cambridge.
In normal circumstances it would be a no brainer to split it. The Science Park was a big boom area and the college is always going to be popular + the local transport types/busybodies have been calling for better links to North Cambridge. So if you are going add those to a route from St Neots and Cambourne area then it makes sense to just "use" the X5 as a base to build rather than duplicate with a brand new service.
However, at weekends the X5 was very well used on the long distance flows and they were paying £15/25 a pop. They'll lose that (if it was ever to come back) and it's not clear how many people will return to commuting between St Neots and Cambridge. So it's just as well the Mayor is funding it...