What is the bit of legislation that requires the wearing of high vis jackets when airside? If anyone has an exact reference it would be very useful.
There isn't one. The reason usually cited is Elfin Safety, which generally tends to show that the relevant person doesn't know how to carry out a proper risk assessment and is more concerned with @rse covering than achieving a safe working environment. The main danger of being on foot when airside is of being run over by a baggage tractor hurtling out of the gloom under the terminal at LHR or LGW, which is hardly a risk factor at most GA fields.
The "bible" is
CAP 642 Airside Safety Management
It says
g) All push-back crew members should wear high visibility garments in compliance with current standards;
High-visibility clothing
6.9.8 Irrespective of other measures that are taken to provide a safe environment for personnel working in airside areas, all personnel who will be working outside (i.e. on foot) on the movement area should wear high-visibility clothing.
Note that this refers to people WORKING, which is why airline staff wear the jackets when escorting passengers on the apron but the passengers are not required to.
It also suggests that hi-vis should be worn during low-vis operations, again not a factor at most GA fields which are not equipped to operate in low vis conditions