(Apologies for the lengthy post below…)
Firstly thank you everyone for taking the time to offer such helpful, constructive, and thoughtful comments. I am very grateful to you… SF at its best…
Secondly, we unfortunately have a fait accompli.
After posting my question first thing this am, (the farmer literally only called us about dealing with the pipe last night) I went for a bike ride, and came back to find the farmer, his wife, son and grandson, plus tractors, diggers etc all poised, ready for action. I felt thoroughly boxed in as I wanted to properly review the SF comments before deciding on a course of action, but felt there would have been a huge loss of goodwill to simply send them away.The comments though did enable me to raise many of the points with the farmer - in real time. I should stress that the attitude of the farmer was characteristically friendly, quite the opposite of threatening etc.
To take the various helpful suggestions, it would apparently cost something like 1500€ minimum to pay to run a pipe 150-200m from the water mains in the road to a suitable location in the meadow. That appeared to be out of the question when I raised it…several times!
He agreed that this was purely an arrangement between us as individuals NOT between two neighbouring properties. Obviously not a guarantee of avoiding creating an easement but I will document the discussion, for what it’s worth.
We agreed that we would charge him for his use only at cost, per the bills, specifically for the itemised consumption-related charges, including the various amounts for action against pollution etc. There can’t be an actual profit element if solely at cost, you’d think, but there might be an argument for disclosing the arrangement on a return, albeit it is hard to see the fisc being terribly interested if there is no profit/profit motive. He thought we might be looking at about 200-300€ a year in cost reimbursements.
I asked about potential water restrictions.He said this part of Normandy has never - to date(!) experienced restrictions whether on farmers or individuals, other than filling swimming pools. He thought it unlikely there would be such restrictions but understood that in theory it might stop us from supplying him if we were facing restrictions due to increased usage, or extra costs. He added that Veolia might send us an automated letter pointing out our consumption has doubled etc, ie do we have a leak etc. He has received similar (so have we) and says there is zero follow up.
We discussed the differing approaches that French and Brits might take . I explained that the advice I was receiving was that reselling water was illegal (thank you @Stella), and the arrangement was not really advisable etc. He smiled and pointed out that he’s just asked our other (French) neighbour whether he would be open to a similar arrangement, and the neighbour leapt at the idea. He said that he has several existing arrangements to share water bills with neighbours at his main farm, which have apparently been going for several years without any issues arising. I doubt any are documented via notaries, or indeed by anybody seemingly!
He suggested that the common reaction of French people - not necessarily the authorities(!) - to such an issue would be the classic shrug. Perhaps I am being too British about it. I admit to being uncomfortable with some aspects of this, but have hopefully tried to draw the worst of the sting from the fait accompli…thanks in large measure to your comments.
If someone can tell me which box on the tax forms for 5 kilos of strawberries, 10 kilos of tomatoes, many bottles of homemade cider etc I can ensure the fisc knows the situation 
