The official advice from the NHS is that cough medicines don't work:
"Although some people find them helpful, medicines that claim to suppress your cough or stop you bringing up phlegm are not usually recommended. This is because there's little evidence to suggest they're any more effective than simple home remedies, and they're not suitable for everyone. . . . A homemade remedy containing honey and lemon is likely to be just as useful and safer to take"
http://www.nhs.uk/conditions/Cough/Pages/Introduction.aspx
However it's unlikely that an over-the-counter cough medicine would be a problem if you're considering taking Night Nurse unless, possibly, you're referring to Covonia Cold & Flu Formula which, like Night Nurse, contains paracetamol. (There are 11 different 'Covonia' products).
Possibly useful (instead of the Covonia!):
http://www.simplyrecipes.com/recipes/honey_and_lemon_tea/