Whilst heritage Spanish has been widely examined in the USA, less is known about the acquisition of Spanish in other English-dominant contexts such as the UK, and studies rarely assess the baseline grammar that heritage speakers are exposed to directly. In this study, we implemented a semantic interpretation task to 17 bilinguals in the UK to investigate child heritage speakers’ and their parents’ comprehension of the preterite–imperfect aspectual contrast in Spanish, an area of known difficulty. The results show that the parents are consistently more accurate in accepting and rejecting the appropriate morphemes than the children. Further analysis shows that children’s accuracy was best predicted by age at time of testing, suggesting that young heritage speakers of Spanish in the UK can acquire the target grammar. However, this general increase in accuracy with age was not found for the continuous reading of imperfective aspect. This finding implicates a more nuanced role of cross-linguistic influence in early heritage speakers’ grammar(s), and partially explains greater difficulty with the imperfect observed in production studies with other heritage speakers.