Joint experiments have been conducted on JET and DIII-D to study m = 2, n = 1 Neo-Classical Tearing Modes (NTMs). Very similar instability behaviour is observed on both machines and the scaling of the mode island width with β and the observation of n = 1 islands at q = 2, strongly points to these modes being driven by neo-classical effects. Similarity experiments using closely matched non-dimensional parameters (plasma shape, aspect ratio, q, ρ* and collisionality) on JET and DIII-D show similar global β-limits for the NTM. Further scalings, which combine broader data sets from JET and DIII-D, show an approximately linear scaling, or slightly stronger, with ion gyro-radius and a relatively weak dependence on collisionality.