could someone tell me how much deadheading there was at the time as a proportion of hours flown?
It all depended on the trip and the route. Where there was no direct BOAC service for example HKG - SIN crews would sometimes deadhead on CX. I did it a couple of times. As previously described there were trips where you would deadhead out and back to the Far East to cover for a 'National Girl' As the Days Overseas Allowances (DOA's) were based on meals at the hotel you would loose out because you will have had meals on the aircraft. The advantage there was duty free shopping.