I am currently in full time employment, but looking to return to University in order to complete a Masters in Computer Science in 2018, so I have plenty of time to implement your suggestions here.
My Bachelors was in an Economics related field, but I only managed to achieve a Third, due to personal reasons and a general lack of passion for the subject area, I should have done a Bachelors in Computer Science, but hindsight is 20:20
I graduated around three years ago and have been in positions with a heavy programming element, having implemented some projects using OOP and T-SQL on a regular basis in real life applications. I believe I will be able to handle any of the programming requirements of any such Masters in the Computer Science field because of this.
Obviously, institutions requiring a strict degree level are closed to me, but many institutions I am interested in state that work experience will be taken into consideration.
How do I best present my experience and attitude to these institutions?
Edit: Thanks for all the good advice so far, just to add that I am UK based, but you have given truly actionable steps so far.