Aaaarrrgghhhhhhhhhhhh.
Found it.
My previous fix isn't required unless you want to save some SRAM but the problem was this line in _morsetab:
1, //ASCII 92 \
The backslash at the end of the line is treated as a line continuation, so the comment is continued to include (and therefore remove) the next line. This has the effect of removing one element from the table and so 'a' will be indexed into 'b', 'b' into 'c' etc.
Just change that line to:
1, //ASCII 92 '\'
Pete