Finely grate the zest from the lemons. Halve the lemons and squeeze out the juice. Strain the juice and measure out 115ml.
Mix the cream, lemon zest and sugar in a saucepan. Bring to the boil, stirring occasionally until the sugar has dissolved. Reduce the heat and simmer for 3 minutes. Remove the pan from the heat and whisk in the lemon juice, strain the cream into a jug, pressing the zest in the sieve to extract as much flavour as possible. Discard the zest.
Allow the mix to settle for 5 minutes before skimming the froth of the surface (optional). Pour equal amounts of the lemon posset into glasses. Leave to cool. Cover the glasses with clingfilm and refrigerate for 24 hours to set.