I am completing postgraduate diploma and could/should do the dissertation now to get the MSc. Meanwhile I have been told I could directly apply for a PHd (I have also several years of job experience) Any advice ?
Thanks a lot

Go for the PhD if you have been advised to do this - better job prospects and pay.
You can build on the MSc.

I'd say get the MSc as it looks better than a postgrad diploma, but try to write your dissertation as a possible chapter for your PhD