Forensic examinations involving the comparison of soft plastics, such as cling film and resealable bags, often conclude at the identification of the polymers, with few cases leading to further discrimination using traditional instrumentation. Isotope ratio mass spectrometry (IRMS) can strengthen comparison evidence by using the stable isotopic compositions of the polymer. To define the variability of the material, to inform forensic comparisons and interpretation of evidence, we have examined the homogeneity of rolls of cling film; ten rolls from eight different brands were measured. All films studied were shown to be isotopically homogeneous, with δ13C standard deviations being less than 0.08 and δ2H standard deviations being less than 1.1. A second experiment examined variability within manufactured batches; IRMS results were obtained for seven brands of seven rolls with common batch numbers. The results showed six of these batches to be isotopically homogeneous whilst the seventh batch, a WOW Select brand, showed two different clusters in the results.