From Arabian Assignment: Operations in Oman and the Yemen, by David Smiley. (The Extraordinary Life of Colonel David Smiley Book 2; Sapere Books, 2020), Kindle pp. 176-179:
When I returned to the Yemen in November, 1963, I went in through Aden and Beihan; by then I had met Johnson and Boyle, who had informed the mercenaries in the field of my impending visit. The first of the British to arrive there was Major Johnny Cooper, who had commanded one of the SAS squadrons that served under me in the attack on the Jebel Akhdar in Oman. Shortly after that operation he had left the SAS, having reached the age limit, but returned to Muscat as one of the Sultan’s Contract Officers and did extremely well. He later became the first of Johnson’s recruits. At the time of my arrival in Aden he had already established his headquarters with a wireless set and operator in the Khowlan area, not far from Sana, with one of the princes.
It is worth recording that at the height of the mercenary effort, when I was commanding them, they never numbered more than 48, of whom 30 were French or Belgian and 18 British. They were broken down into small missions — usually one officer, one NCO wireless operator, and one NCO medical orderly — and deployed according to the wishes and needs of the Royalist commanders. It is important to realize that none of the mercenaries actually fought in the war; their job was to advise the commanders, train their troops and provide communications and medical services. The medical situation in Royalist areas was particularly desperate; there were virtually no trained doctors. Until quite late in the war the International Red Cross operated only in Republican territory; but even when it sent a mission to the Royalists its hospital was situated a long way from the fighting and the doctors spent most of their time treating the local civilians for endemic diseases. This was no fault of the Swiss doctors, who would gladly have served at the front, but of the Red Cross directorate, which gave them categorical instructions not to go near it.
I flew to Aden on 14 November, and on to Beihan two days later. There I spent the night in the village of Naqub, twenty miles north of the State capital, in the ‘safe house’ allocated to the mercenaries by the Ruler. I shared it with three Frenchmen, who were in wireless contact with Johnny Cooper and the other missions, and seven British, who arrived in the middle of the night after a drive of three days in a lorry from Aden; in the morning another Frenchman joined us — Colonel Bob Denard, a veteran of the Congo who now commanded all the mercenaries in the Yemen except the British. His Frenchmen and Belgians, though very polite to me, were seldom chatty or communicative outside their own circles; some of them, I knew, had belonged to the OAS and so had little love for General de Gaulle, but I never discussed politics with them. Their attitude to the work was strictly professional; they were there for the money, but they meant to give good value in return. Most of them, as I have said, had seen service in the Congo, and many of them alternated between the Congo and the Yemen, serving now in one theatre, now in another. The reason, I discovered, was that in the Congo they had all the drink and women they wanted, but seldom received their pay; whereas in the Yemen they had regular pay but no women or drink. And so when they had earned enough in the Yemen they went off to the Congo to enjoy it.
The British, on the other hand, were more often inspired by enthusiasm for the Royalist cause or a simple thirst for adventure, although there were some deplorable exceptions — one fairly senior officer, in particular, was strictly on the make; unfortunately mere enthusiasm was an unreliable guide to efficiency, and I discovered later on that, while the NCO specialists did excellent work, the British officers who proved their worth were those who understood some Arabic.