Costa put all their nutritional info on their website and the syns would be 1 per 20 calories. If you are happy there is enough milk in it to count as a HEA then you can take 6 syns off that
I think it may have been the custard cream biscuit that did the damage lol. Costa are fairly low syn for coffee if you go for the skimmed milk.
It's much easier if you just decide not to eat anything without first knowing the syns - avoids any nasty surprises