[ECML43]

I worked out when I first started trainspotting that all british trains have to have a patch of yellow on both driving ends to make them more visible to other trains, but what is the rule about it (if there is one). Because the Grand central 43s have only a small strip of yellow below the cab window, whearas most MUs have the entire cab front covered in yellow. Looking at old photos it seems to me like the rule was invented by BR in the late 60s. But what are the specifications of this 'rule' on yellow cab fronts?