The new ram pump had been working for 3 weeks. During this time we tried running the pump to a tank situated above the garden, so that we would be able to water the garden through a gravity-fed hose. The pump delivered the water fine, though the friction of running water through a long length of garden hose prevents a decent flow. The solution would be to run fatter pipe to a hose point and then hose only as far as needed.
However in the meantime we had to switch the delivery of water between this new tank and the original location near the kitchen, to allow refilling of drinking water. The switching involved a gate valve installed near where the pipe passed the kitchen. To switch over, I turned the valve closed and quickly remove the lower end of the hose which could then be moved to where needed. This manual system seemed to be ok although I did wonder if turning the valve closed first could cause a pressure build up in the delivery pipe.
In any case it seemed to have proven the concept so the WWOOFers buried sections of the pipe that ran across the land where people might step on it.
When I was next onsite they told me the pump had stopped, and that when they had opened the connection near the kitchen, no water had come out.
Investigation: first I opened up connectors lower down the hill; none had water, so the water had leaked out somewhere lower downhill. I was able to re-start the pump but nothing came out.
At the pump, I found that the connection to one of the pressure guages was leaking, and also noticed water coming out of the waste valve when the intake was shut but the delivery pipe valve was open. This made me suspect the one way valve, so I removed it to have a look. It is metal but has a plastic part in it which had cracked and was allowing water to flow backwards. I replaced the whole valve yesterday with an all-metal valve, no spring, and was easier to start up. Flow with the new valve is about 1 litre / minute compared to the spring valve which was about 1.5 l/min, but that's still more than I was getting with the PVC pump.