What can I do to " fresh cream" to substitute "double cream"?
In NZ normal pure cream is close enough to "double cream" in performance, it's around 35% to 40% fat. By general global definition it should be called "heavy cream"

NZ cream is pure skimmed cream from fresh milk, and does not need thickening or other processing to produce excellent results as a whipped product, or for a thick cream additive in sauces or fillings

Cream can get a little seasonal just after calving time (Aug), but in general it's just perfect

The export variety is often labelled "Culinary Cream" and is 38% fat or higher. This is what many people around the world would consider NZ cream (other than NZ butter or AMF)

If you want closer to 50% fat cream, ask your local farmers for some raw milk and skim out the lumps of fat from that. Let the milk settle overnight and skim the cream from that. Then gently heat the fat lumps to melt and carefully mix into the skimmed cream. Not for the faint of heart!

NZ "Thickened cream" or "Mascarpone cream" is standard pure cream with a thickener like E1442 added, you can make your own too