The VO wasn't convince that there is a career progression in your selected program or whether it would emanate to a self development.
Going for an Advanced Diploma after a Bachelor isn't a good idea and shows no career or educational progression. I see you've change to a PGD which is fall better than an Advance Diploma. However, going for Master is the most suitable progression, but a PGD after a Bachelor is also a good point of progression from a professional viewpoint. I think your on track applying for a PGD.
Its advisable to write a very convincing and well crafted SOP by trying to convince the VO why a PGD instead of a Master, and try to connect it to your career progression and self development. Do a proper research about your prospect course and it benefits to your future reaseonableness and endeavours. In it, extensively discuss your home ties from an economic and social viewpoints with subtantial proof
Good luck!