Some notes that I'm reading right now show a picture of the distribution of pressure around an aerofoil with zero angle of attack. The notes say that the pressure is identical at the top/bottom surface but is below the local ambient pressure and the pressure at the front of the aerofoil is above the local ambient pressure. They add in after that the length of the arrows denote the relative magnitude of the pressures but the top/bottom surfaces have longer arrows despite them stating the pressure is lower? Is the relative magnitude of the pressure at the top and bottom surfaces actually greater or is this just a misleading picture?
If you are looking at a diagram with arrows, no doubt they represent the flow velocity (which is a vector), rather than the pressure (which is a scalar).
Higher speed (the magnitude of the velocity) corresponds to lower pressure.
The flow typically starts at zero speed at the leading-edge stagnation point, accelerates to a maximum at some position farther aft (depending on the shape), then decelerates to another stagnation point at the trailing edge (in some idealized cases), or to something close to the freestream value (for most other realistic cases).
Could it be that you looked at the result of an XFOIL simulation? There, small arrows are indeed used to indicate the local pressure; suction is indicated by arrows pointing away from the surface and vice versa.
Pressure distribution over an E502 mod at 3° angle of attack using arrows, courtesy of XFOIL.
The notes say that the pressure is identical at the top/bottom surface
Only if the airfoil is symmetrical. Most airfoils have camber in order to improve their efficiency when creating lift.
Is the relative magnitude of the pressure at the top and bottom surfaces actually greater […] ?
The displacement of air by the airfoil means that suction is needed to bend the flow around the airfoil's contour, and with thicker airfoils this suction can indeed be substantial. You will note in the plot above that there is still some suction on the lower side even though the airfoil creates positive lift, has some camber and a positive angle of attack.