Meanwhile, roll out a piece of plastic wrap on which you lay a regimented row of thinly sliced back fat as long as your terrine or bread tin. Lay a lengthways row of your soaked prunes two-thirds of the way up the row. Then, using the plastic wrap to lift the fat, fold the top third back over the prunes, press down, and remove the cling film. Then, again using the cling film, bring the other two-thirds of fat over the almost covered prunes, then gently roll the whole thing back toward you, at which point you should have a perfect prune/fat roll. Remove the plastic wrap.