What is a good substitute for heavy cream in chowder?

Because the recipe also uses water and white wine along with the cream, I wouldn't automatically assume that the broth would be "too fatty and rich." I suggest making the recipe as directed and see how you like it. You could add a bit more water at the end if you felt the need to.

How about pressed plain yoghurt, or plain greek yoghurt?

I've used sour cream in a pinch, but the fat content may be similar. There's always low fat sour cream but I have my doubts about the actual health benefits of that, and besides, it may not work in this soup.

Or mix heavy cream, half cream half water/lowfat milk/milk substitute.

Personally I would go with the yoghurt. The process of culturing yoghurt changes the protein and fat molecules slightly, in a way that makes it easier to digest and get the nutrients out of - unlike cream on it's own which can be more difficult to digest.

Let us know how it turns out.

I agree - the classic chowders I've had in New England typically use milk and broth. The starch from the potato gives it a little body. Heavy cream seems like it might mask the briny intensity of the (hopefully large) pieces of clam in the chowder. I think of it more for, e.g., a bisque, where you want that thick, rich texture. (No health related qualms, it's just not the classic chowder texture/taste for me.)

Old-timers often used evaporated milk, as it was shelf-stable and it doesn't curdle. I find it very satisfactory myself. I'd add it after simmering the potatoes (if that's what's in the recipe) in broth or water. Heavy cream seems awfully, well, heavy for a chowder.

Well, I'd use the cream and enjoy it. You don't eat it every day, you might be able to get more than four servings out of it, and I'll bet it isn't bad for you anyway. If you have to substitute, I'd use whole milk--NOT low fat or skim.

You can use half-and-half or whole milk. If it feels to thin to you, make a little roux of flour and butter and add that to thicken things up.

Evaporated milk can be a stand-in for heavy cream. For a creamy mouth feel, you could put one pat of butter in each bowl before you serve the chowder.

Simmer a couple potatoes in milk (preferably whole milk) and puree them to serve as the base. I would use maybe 1 or 2 peeled yukons (depending on size); too many and it will make a starchy and gummy soup.
OR, cook up a small batch of rice (like a 1/2 cup COOKED) and puree that in.

I'd still add a good splash of cream at the end with either of these suggestions for the richness and textural, fatty mouthfeel.