Need assistance with IF function formula

Hopefully someone can resolve an issue I have with an IF function formula.

I have attached a file with a a copy of the formula I am currently using (col L) plus the associated conditions. The issue I am having is trying to figure how to exclude a value in column N from the calculation in all cases, even if some of the conditions in the formula permit an answer to appear in column L. (Column I is system generated, and column L validates the result).

The current formula works 90% of the time. It is trying to resolve the issue with the final 10% that is giving me headaches. Also any suggestions for improvements/alternatives would be greatfully received.

rgds,
Cymraeg