depends on the brand of lager (they vary hugely), if you know which it was, a quick google will normal give the nutritional info and it will then be 1 syn for every 20 calories, as no free food allowance.
the blackcurrant will be synned too, as it will most definately not be sugar free. Again that is 1 syn for every 20 calories too.
A pint of lager though is about 170 calories - 340 calores total
the cordial for a dash in two pints would prob be about 50 calories.