Place the water, sugar and tarragon into a medium sized pan and bring to a boil.
Cut the rhubarb in 2 cm pieces.
When the liquid starts boiling add the rhubarb and continue cooking for 2-3 minutes, until the rhubarb begins to soften.Take a few pieces of rhubarb out, allow to cool and chop them.
Leave the rest of the rhubarb in the pan, covered for about 20 minutes, until it softens completely. Then take it out and blitz it once in a food processor. Add the chopped ginger and the rest of the chopped rhubarb.
Serve with scones and clotted cream.