Isn't that a six-coach set, including the DBSO (i.e. five Mk3s)? My understanding is that the formation of most sets was five carriages - the DBSO, 3xMk3a TSO and 1xMk3a CO (not sure which end of the set the CO usually was). Is this not correct?
If this is right, then the 4-coach set as proposed, plus the DBSO and 47, is the correct formation for most of the trains and an additional TSO is not required.