You can make this wine with stones in as its the easiest way.
Wash the wild plums.
Wild plums are open to infection of worms and other bugs, so check each plum individually.
Add the plums which you have mashed by hand to your fermenting bin, discard any suspect wild plums.
Add sugar to 1 pint/0.5 litres of water and bring to the boil, add this to the bucket while still very hot.
Add 4 pints/2.25 litres of cold water, stir and wait to cool.
Now add citric acid, pectic enzyme and tannin, stir well cover and leave for 24 hours.
After 24 hours from adding tannin, citric acid and pectic enzyme.
Add yeast and yeast nutrient into the must, leave for 2 days stirring 2 or 3 times a day.
Strain must into a demijohn, removing large solids beforehand.
Add the grape concentrate of choice and top up demijohn with cold water.
Fit airlock and move to warm place, for the first few days give it a stir daily.
When fermnetation finishes add a crushed Campden tablet.
Move to a cool place to settle.
Leave one week then rack if necessary.
When fully clear bottle and maybe sweeten to your taste with a non-fermenting sweetner.
Enjoy this wine in 6 to 12 months.