Here's my two cents:
It would be helpful to explain in your SOP to the consular why a Post graduate Diploma is the natural progression after the HND and say not a bachelors degree or masters. Also include why you are choosing that particular course? why you are choosing to do it in Canada and not your home country.
Is anyone sponsoring your studies? if they are ask them to write a sponsorship letter to attach to your documents. if you are working now, get a letter from your employer stating that you work there and perhaps include in your SOP as well that you are going to return back to your home country to continue working with your employer.