Thank you, Wintergoose, for your detailed reply.
I do have a syphon pump - an IBC which came with a brand new Alderice brass pump and packing box, which came with my mill
My project is to pump from a well, dug into the old sand bed of a creek that is now relocated itself about 100 yards away. The water table in the well seems pretty consistent with the level of the active creek, which flows on the surface all year round, normally, but has sub surface flows in the summer if the surface flow stops. The mill is to lift the water into a tank about 100 yards away, with about 30 feet lift/head in total.
My pump jack is fully enclosed and hence I am not sure about extending the discharge pipe above it to a spill pipe? My plan is to cap off either the tee piece which is part of the pump jack (it is at the base plate) or the outlet from the syphon pump. The down stroke of the mill will force the water into either of those outlets, and hence to the tank.
I note your comment about the need for a swivel on the pump rod from the mill head - a point I had overlooked, but then, is there not one built into the pump motor? I need to investigate that.
Having said all this, I am doing this for fun! We now live on the edge of town, with town water, which probaly will cost a fraction of all this energy and worry! Still, it gives my wife and daughters something to jibe me about, when they are not questioning why I would be restoring two Versatile tractors!