Also surprised the westerly approach over London is shorter than area covered in the west for easterlies.
Most of the lobes on the contours correspond to the NPRs, i.e. for departing traffic, so it's reasonable to expect a larger area to the west.
The footprint also misses the approaches for the cross runway (which was still in use)?
Yes, Runway 2 was certainly in use in 1974, mostly for landings on 23. But the Lden contours are a weighted annual average value, so the relatively small number of movements on that runway would have very little effect on the noise footprint calculation.
Good intro to noise contours here:
Measuring the impact of continuous noise | Noise | About the CAA