Thanks to Tom Herbet who spotted my mistake and replied privately
1 B-TREE.
3 FILLER PIC XX VALUE "[]".
3 FILLER PIC S9(4) COMP SYNC VALUE 0.
3 FILLER PIC S9(4) COMP SYNC VALUE 6.
3 FILLER PIC S9(4) COMP SYNC VALUE 6.
3 FILLER PIC X(12)
VALUE "CA000 DA000 ". << client key is 6 bytes of format AAnnn + space >>
The use of "COMP SYNC" was introducing slack bytes - changing the defintion to plain old COMP fixed the problem.
Ta again Tom !