Mid september was the date I heard for the full introduction of the Class 180 service, and that the halts train would be formed of a Class 180 coming through direct from Paddington as the 16:22 train. There will, I'm sure, be a phased introduction on other Cotswold Line stations as of early summer.
It appears that the 'Halts' train coming through from London has been shelved for now at least. Journey planners show it starting at Didcot at 17:15 from the December timetable change.