how many hours does it state on your permanent contract?
you should have a copy of this, if not request it from manager.
were you on full time hours at some point in the past?
did you seek to reduce your hours at some point in time?
you can have a permanent contract for any amount of hours up to 39 per week, so if when you went on 2 days a week, your contract might have been reduced accordingly, I am aware of this happening in the public service, without staff realising the consequences of reducing their hours, i.e: when they wanted to return to full time hours they could not.
If your contract says 2 days a week, then i'd say at the moment that's all you'll get.
I'm not an expert, just speaking from experience.
And as for returning to full time straight away, asuming you can, where I work in the public service, it's highly unlikely.
Try and talk to someone informally first to find out exactly where you stand before approaching management.
Good luck.