I would suggest that you use a systemic insecticide such as Bayer Provado Ultimate Bug Killer Ready to Use to reduce the aphid population.In winter use an oil winter wash such as Growing Success Winter Tree Wash to remove overwintering aphid eggs. This winter treatment is likely to be most effective at reducing the problem for next year but you will need to be vigilant in spring for a recurrence of the aphid which will need spraying. If the problem recurs next spring you could try using an organic spray such as Bayer Organic Pest Control. This has quite a short persistence and works on contact so you will need to be thorough and may have to spray several times throughout the season. However, this spray can be used right up to a day before harvesting which is a great benefit.