I just took a look at the camera and discovered I accidentally changed the wrong setting at some point - exposure compensation was at +3, which definitely explains the over exposed images. Both suggestions made are appreciated and will be implemented, thank you.
EC doesnt do anything in manual mode though unless its flash EC with an on camera flash
I might be wrong on this but i think EC will still affect the ISO in manual if you have it set to auto ISO.
Yea, if you're in M with Auto-ISO it'll crank the ISO up to try and meet the +3 EV that was selected.