In Silverlink days there was talk of a dedicated fleet of 313s to be found once/if the Bricket Wood passing loop was installed for a half-hourly service. With the franchise change and new 350 order I'd imagine that will have all changed - Watford men must be 321only now unless taking refreshers on 313s as I'm pretty sure they still only have Northampton and some Bletchley men signed on 350s.