What do you want to do with your Master's? You know you got a 3rd and can't apply for any grad schemes. Might as well go into the labour market with a PgDip and see what happens. Just try as hard as you can to get a job. Start from the bottom (PT jobs, temping, occasional work, unpaid/paid internships, but not volunteering - this is classified as volunteering in your CV, not as work experience - more of something you do for pleasure) and try to climb up. If you think you hit the rock bottom because you can't find any job, you are not there yet. There will be a time when you question your existence at all - because you can't find employment, and you think "anything that saves me from this hell," then do a Master's.
I really do not see the point in doing a Master's degree unless you know exactly what you want out of it.