To make the chamomile cordial, infuse chamomile tea bags in sugar syrup - two parts sugar to one part water - and add a heaped teaspoon of citric acid or the juice of three large lemons. An alternative mixer would be elderflower cordial served with a chamomile tea bag... trust us, it works really well!
Add all ingredients to an ice-filled highball and top with soda water. Squeeze the oils from a sliver of lemon zest and discard. No need to garnish.