Answering my own question - I have been sent the following list by Delta Tau, who stated that they will include this in the next edition of the Software reference manual.
3 SYSTEM FILE NOT AVAILABLE
20 ILLEGAL CMD
21 ILLEGAL PARAMETER
22 PROGRAM NOT IN BUFFER
23 OUT OF RANGE NUMBER
24 OUT OF ORDER NUMBER
25 INVALID NUMBER
26 INVALID RANGE
31 COMPILE ERR
32 BREAK POINTS SET
33 BUFFER IN USE
34 BUFFER FULL
35 INVALID LABEL
36 INVALID LINE #
37 INVALID BRKPT
38 PROGRAM RUNNING
39 NOT READY TO RUN
40 BUFFER NOT DEFINED
41 BUFFER ALREADY DEFINED
42 NO MOTORS DEFINED
43 MOTOR NOT CLOSED LOOP
50 MACRO COM TIMEOUT
51 MACRO PORT NOT OPEN
52 MACRO RING SELECTED NOT AVAILABLE OR PPMAC NOT SYNCH MASTER
53 MACRO NOT AVAILABLE, NO MACRO ICs
54 MACRO ASCII REQUEST EXCEEDED BUFFER SIZE
55 MACRO ASCII COM TIMEOUT
56 MACRO RING INTEGRITY IN FAILED STATE
57 MACRO SYNC MASTER MUST HAVE STN=0
58 MACRO ASCII COM IN USE BY ANOTHER THREAD
59 MACRO MRO FILE OPEN OR READ ERR
70 Struct Write Data Error
71 Struct Write Undefined Gate Error
72 Struct Write L Parameter Error
73 Struct Write Index Error
74 Struct Write Card ID Error
75 Struct Write Error
76 Write To Struct Address Error
77 Struct Write Gate Part Number Error
80 MODBUS SOCKET NOT CONNECTED
81 MODBUS SOCKET BUSY
82 MODBUS SOCKET SEND/RECV ERROR
83 MODBUS SOCKET CREATE ERROR
84 MODBUS SERVER EXCEPTION ERROR
85 MODBUS SOCKET IN USE
86 MODBUS SERVER RESPONSE FORMAT ERROR
87 MODBUS SOCKET CONNECT ERROR
88 MODBUS SERVER SOCKET LISTEN ERROR
91 MACRO STATION: ILLEGAL(I,M,P,Q) DATA_TYPE
92 MACRO STATION: ILLEGAL(I,M,P,Q) DATA_NUMBER
94 MACRO STATION: REMOTE COM TIMEOUT
95 MACRO STATION: ANOTHER STATION AT THIS ADDRESS
96 UNKNOWN # & ERROR