Concealed cistern stopped working - how to access?

My loo won't flush but it's difficult to diagnose the problem as the cistern is concealed behind a tiled wall and impossible to access. The cistern is operated by a push button mechanism. I've unscrewed this but the hole behind it isn't big enough to access the cistern itself. Could be the problem is with the button itself or the air tube that's attached. Shouldn't be worn as it's only about 18 months old. Am I going to have to remove tiles to access or is there a trick I'm missing here? Suggestions would be gratefully received. Cheers.