Going from STEM to history I am looking to leave my career in computing and go back to university to study a History MA.

The trouble is I only have a 3rd class degree in Computing from around 1990.

What would you recommend I do to get myself back in the saddle, and to make myself more attractive to MA courses?
I wouldn't expect your degree in computing from three decades ago to have much impact on your application for a postgraduate degree. You can confirm by contacting a perspective university. (I doubt university websites will be particularly useful, since they probably won't consider your situation.)