Exeter to Plymouth (westbound) is sort of half clock face, half not, as the hourly westbound XC services generally leave Exeter at 45 or 47 past the hour and the hourly westbound FGW's from London seem to vary between 10 and 35 past the hour depending on number of stops between Paddington and Exeter, with trains generally leaving Paddington at 06 past the hour, the only exceptions being the odd ones routed via Bristol and the few slow 'stoppers' via the B&H to Exeter.
In the opposite (eastbound) direction, it is pretty much clockface, with the XC's leaving Plymouth at 24 or 25 past the hour and the FGW London trains leaving between 55 and 00, however the last train of the day for the Capital (sleeper excluded) leaves Plymouth at 1942 (1739 from Penzance) but is routed via Bristol.