Had a quick look at a random date in March and it seems on weekdays the flights are as follows:
EXT-LCY 0645
EXT-LCY 1510
However the LCY second flight doesn't leave until 2030 so one assumes the aircraft will serve one of the additional BHD or EDI in between. Still useful for those in London on day trips but for the few (and I expect it to be just a few) on day trips to Exeter it's not exactly a long day when considering journey times to Exeter airport from the city centre.