Pre-heat the oven to 160°C and line a 900g loaf tin with baking parchment.
Cream the butter and caster sugar in a mixer until pale and fluffy.
Sift the flour and baking powder into a bowl and mix with the ground almonds.
Beat the eggs and fold them into the butter mixture gradually, beating well after each addition.
Pound the lemon zest and thyme leaves together with a pestle and stir into the mixture.
Gradually stir the flour and almond mixture into the wet ingredients.
Spoon the batter into the tin and bake for 45 minutes.
Dissolve the sugar in lemon juice over moderate heat and stir in the remaining thyme.
Prick the warm cake surface with a skewer and pour the lemon syrup over it.
Cool the cake completely before slicing and serving.