Problem with FLIRTs is that they waste platform length, upside is (particularly on a DOO railway) level boarding. Will be interesting to see.
Do they waste as much platform space as a loco and DVT?
As far as I can see, the only Chiltern stations that can't accommodate anything more than a 3-car FLIRT without SDO is Sudbury & Harrow Road and the bay at Princes Risborough. A few stations would be limited to a 4-car FLIRT – Sudbury Hill, Northolt Park, South Ruislip*, Great Missenden*, Wendover*, Stoke Mandeville*, Monks Risborough, Little Kimble
(* in one direction only).
All other Chiltern stations can accommodate a 3+3 FLIRT at least, equivalent to about 4.5 carriages of traditional stock – and given how many 2 or 3 carriage trains Chiltern run normally, I don't think that they are close to needing to worry about whether their trains are too long for their stations!
Is battery technology well-enough developed to allow a battery-only unit to be specified? As the 165s are used predominantly on stopping / suburban services, would this be an option or would distance (eg Aylesbury - Marylebone or Marylebone - High Wycombe) be beyond the limits of battery capacity?
It isn't just about battery capacity, it's also about charging time. If part of the route is electrified then trains can recharge their batteries while running on the electrified section, meaning less downtime. But if little or none of the route is electrified then you have to factor in enough time to plug in and recharge at the terminus, which could well mean extended layovers and a much less efficient utilisation of resources – ie, you would need 3 trains to run the same frequency of service that you can currently do with 2.