Begin with the rosehip coulis. Wash the hips then top and tail with a sharp knife. Cut them in half lengthways and remove the seeds by scraping with a teaspoon, including the fine hairs. Place in a small saucepan and add 180ml of water, bring to the boil, reduce to a simmer and particially cover with a lid. Simmer for 20 minutes, or until the hips are soft.
Remove one third of the hips and a 2-3 tablespoons of the liquid. Place into a fine sieve over the saucepan with the remaining cooked hips. Mash and push them through the sieve with a spoon. Discard what is left.
Add the sugar and lemon juice and place back on the hob and simmer for 10 minutes, or until the syrup thickens. Remove and cool. Place into a jam jar and pop it into the fridge until its needed.
Soak the gelatine leaves in cold water. Pour the cream into a saucepan, add 80g of the sugar and heat gently, stirring, until it's dissolved. Bring just to a simmer, and then remove from the heat.
Squeeze out the gelatine and then stir it into the hot cream mixture to dissolve. Pour it through a sieve into a clean bowl and stir in the milk and buttermilk. Taste, add a little more sugar if you like it sweeter.
Have four moulds, cups or ramekins ready and pour the mixture between them. Cool and then refrigerate for at least 5 hours or better still overnight.
To turn out, put the moulds briefly in boiling water and then invert on to plates. Serve with the roseship coulis.