The "hardly attractive" 0736 train would actually be more attractive should it be extended to start from Matlock. Currently if your a Derwent Valley Line commuter to London your only option would be the 0619ex Matlock and changing at Derby onto the 0705 which gets you into London at 0856, a total journey time of 2hr37min.
Now if the 0619 ex Matlock ran a few minutes earlier throughout to leave the branch at Ambergate a few mins earlier, this would allow an ecs 222 to reach Matlock by 0650 and say that departed at 0655, arriving Derby 0730, departing 0736 as is now, arriving London 0946, this is a total journey time of 2hr50min (only 13 min extra, without a change). This could also be reduced to 2hr31 should they wish to change at LEI (announcements regarding this are made by the TM).
EMT wouldn't need to run the Ambergate journey, however, the 0652 DBY - MAT would need to run around 5 min later throughout to facilitate the 222 to come off the branch before that could join. This wouldn't have an effect on the return working as there is enough turnaround time at Matlock. Crew wise this wouldnt be a problem, the crew who do the Ambergate run could just book on around 15min earlier and run ecs to Matlock, in service to Derby whereby a crew change would take place, the driver and TM that would normally work the 0736 take over and the crew that have just worked it in would go on to work their next service which is the 0740 to Crewe.
All do-able. Likely to happen? Probably not