Hello all. I'm very happy to say my filter is working perfectly. What it looked like before was that I was aliasing input frequencies above Fs/4 the decimating Nyquist. After much trial and error I think my problem was that 1. I didn't account for the filter gain - after filtering - the skirts then were heavily amplified. 2. I may have not added in correctly the previous MAC. (time for version control software) Good luck to all and thanks for your responses !!!