I've under carbed my beer bit of advice please.

[Covrich]

That should have given an adequate level of carbing. I would give the original bottles a swirl to redistribute the yeast and find a warmer place in which to put them. Your yeast might just be taking its time to work at 18*C but might be more enthusiastic at a higher temperature

Exactly that is ample for most styles.. You shoudl be okay just need more time and as terry says maybe a little agitation won't go amiss

 

[Braindead]

GO with the suggestion of turning the bottles upside down for 24 hours then put straight again.
Works a treat