Many parents in the UK enrol their children in Greek supplementary schools so the children can learn and maintain the Greek language and culture in parallel with English mainstream education. Despite fears about the effects of this heritage language (Greek) use on children's skills in the majority language (English), research on these somewhat hidden schools to date is limited and qualitative in nature. The current study is the first quantitative study which examines the effect of attending a Greek supplementary school on the vocabulary and grammar scores of Greek-English bilingual children. We administered a battery of language tests in both languages to 31 Greek-English bilingual children, aged 5-13 years, and closely looked at the participants' language history using parental questionnaires. Using multiple regression analyses we examine the relationship between relevant variables, such as language use and years in supplementary school and we find that the higher the use of Greek, the higher the scores in the Greek language tasks, although no significant relationship was detected between years in supplementary school and the development of language skills. Crucially, the use of Greek does not negatively predict scores in the English language tasks. Implications of our results and future directions are discussed.