The GCR's Diesel running weekend is the 14th & 15th, NOT the 7th & 8th. See this link for details
http://www.gcrailway.co.uk/Events.aspx?ID=397 It's D5830 & D8098 Saturday ONLY and D123 & D5401 Sunday ONLY.
The ordinary timetable for this weekend ( 7th & 8th) does show diesels working 4 trips, between the steam journeys but is not specific as to wether loco or railcar will be used.
Regards, 62440.