OK, first thanks for the full and very helpful explanation and I can now report a good measure of success.
Using your values of -71, -43 -14 for switch E and 0 , +85 for switch F gives me PWM readings of 2100, 1752, 1049 with switch F forward and 1994, 1285 and 928 with it back.
that gives in decending order 2100, 1994, 1752, 1285, 1049 & 928. not quite as evenly spread and the spread is a little large.
I will start to experiment and reduce the offset values and see if I can get nearer to the theoretical readings that I require.
These settings don't give me six mode settings but they do give me four which is one more than I had before!
Can you suggest what offsets I should try next?
Many thanks for your help so far
Martin.