This is the author accepted manuscript. The final version is available from the European Southern Observatory (ESO) via http://dx.doi.org/10.1051/0004-6361/201424188 ; We present observations and analysis of a sample of 123 galaxy clusters from the 2013 Planck catalogue of Sunyaev-Zel?dovich sources with the Arcminute Microkelvin Imager (AMI), a ground-based radio interferometer. AMI provides an independent measurement with higher angular resolution, 3 arcmin compared to the Planck beams of 5?10 arcmin. The AMI observations thus provide validation of the cluster detections, improved positional estimates, and a consistency check on the fitted ?size? (?_s) and ?flux? (Y_tot) parameters in the Generalised Navarro, Frenk and White (GNFW) model. We detect 99 of the clusters. We use the AMI positional estimates to check the positional estimates and error-bars produced by the Planck algorithms PowellSnakes and MMF3. We find that Y_tot values as measured by AMI are biased downwards with respect to the Planck constraints, especially for high Planck-SNR clusters. We perform simulations to show that this can be explained by deviation from the ?universal? pressure profile shape used to model the clusters. We show that AMI data can constrain the ? and ? parameters describing the shape of the profile in the GNFW model for individual clusters provided careful attention is paid to the degeneracies between parameters, but one requires information on a wider range of angular scales than are present in AMI data alone to correctly constrain all parameters simultaneously. ; The AMI telescope is supported by Cambridge University. YCP and CR acknowledge support from CCT/Cavendish Laboratory and STFC studentships, respectively. YCP and MO acknowledge support from Research Fellowships from Trinity College and Sidney Sussex College, Cambridge, respectively. This work was partially undertaken on the COSMOS Shared Memory system at DAMTP, University of Cambridge operated on behalf of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1. CM acknowledges her KICC Fellowship grant funding for the procurement of the cluster used for computational work. In addition, we would like to thank the IOA computing support team for maintaining the cluster.