A well timed 2/3 times daily LCY flight could probably operate happily alongside a daily midday STN flight
There's a practical limitation here. To be useful for business there needs to be a thrice (min. twice) daily service in peak time slots out of LCY. When last tried it was T3 on a J41 which became (more) loss making when the introductory discount offers lapsed and GIP upped the charging regime to put small props at a disadvantage. This pushed the throughput up as BA CityFlyer introduced the Embraers and the volume of F50s and ATRs dropped as a result.
Now flybe are on board, it might be worth re-visiting as if they can't do it, no-one can IMHO.