In a food processor, blend half of the diced cucumber with the wine or cider until smooth. Set aside.
In a saucepan over a low heat, melt the butter and gently sweat the remaining diced cucumber for 10 minutes. Be careful not to let the cucumber brown, and adjust the heat accordingly. Stir in the flour and slowly add the blended cucumber liquid, whisking by hand constantly. Season and continue to stir over a gentle heat for 10 minutes until thickened. Stir in the cream and heat through.
This sauce can be served hot or cold and can also be made ahead and stored. To store, pour the cooked sauce into a screw top jar and keep in the fridge until needed.