Taking co-amoxiclav for my sinus infection, will this also get rid of the post nasal drip?

Yes : Your post nasal drainage is your body's response to the infection. The mucus contains white blood cells called polys that kill the bacteria that are attacking your sinuses. Normally this mucus is thick and colored. Your antibiotic helps your body overcome the bacteria. Your drainage should gradually thin and stop as the infection clears up.