For the pastry, cream the butter and sugar together. Sieve the baking powder, flour and salt together and stir into the mix. Slowly mix in the cream. Wrap in clingfilm and chill for 30 minutes.
On a floured table, roll out the pastry to about 5mm thick. Use to line a 26cm tart tin, about 3–4cm deep; rest in the fridge for 1 hour.
Preheat the oven to 160C/325F/gas mark 3. Meanwhile, make the filling: mix the golden syrup, treacle, double cream, oatmeal and eggs together, then stir in the lemon juice.
Fill the tart tin with the mix, bake for 40–50 minutes, then leave to cool. Serve warm with some good extra-thick or clotted cream.