Increasing use of diagnostic genomic sequencing is pushing health services to confront the issue of opportunistic genomic screening (OGS). To date, OGS has been offered concomitant with diagnostic testing. In contrast, we piloted a service offering OGS after return of diagnostic testing results. Evaluation was designed to provide insights for future models of service and included patient surveys at three time points, semi-structured interviews with genetic counsellors (GCs) and a focus group with medical scientists. Uptake was relatively low: 83 of 200 patients approached (42%) attended the OGS service, with 81 accepting OGS. Whilst many who declined to attend the service cited practical barriers, others gave reasons that indicated this was a considered decision. Despite specific genetic counselling, one third of patients did not understand the scope of re-analysis. Yet after post-test counselling, all respondents with novel pathogenic additional findings (AF) understood the implications and reported relevant follow-up. Recall was high: five months after last contact, 75% recalled being offered OGS without prompting. GC interviews and patient survey responses provide insights into complexities that influence patient support needs, including diagnostic status and AF result type. There was no consensus among patients or professionals about when to offer OGS. There was a clear preference for multiple, flexible methods of information provision; achieving this whilst balancing patient support needs and resource requirements is a challenge requiring further investigation. Decisions about whether, when and how to offer OGS are complex; our study shows the two-step approach warrants further exploration.