Beat in the clotted cream (or double cream) until relatively smooth - a few small lumps don't matter, but you can sieve the liquid if desired.
Pour the ice cream into a suitable freezer-proof bowl, cover and put it in the freezer for 1-1 ½ hours. The sides should be beginning to freeze, but the centre will be soft and slushy.
Remove from the freezer and, working quickly, use electric beaters to beat the ice cream until the ice crystals are uniform. Cover and place back in the freezer.
Repeat this process 4-6 more time at ¾ - 1 hour intervals. Once the ice cream has reached the soft serve consistency you are ready to swirl through the blackberry sauce.
Spoon ⅓ of the ice cream into a clean container and dollop ⅓ of the blackberry sauce on top. Add another ⅓ of custard followed by another ⅓ blackberry sauce and then a final layer.
Using a skewer or a blunt knife swirl the ice cream and sauce together to achieve a rippled effect.
Cover with clingfilm and freeze until set.
Remove from the freezer 15 minutes before serving.