The usual cause of greenback is excessive heat in the greenhouse during the development of the fruit. The risk of this can be reduced by applying shading to the glass in early summer, damping down the greenhouse paths in the morning on hot summer days, and ensuring that there is adequate ventilation. All of these measures will help to reduce plant stress on hot days. Whilst it may be too late this season, we would recommend using all 3 of these measures next time.
Greenback can also be caused by a lack of potassium. Feed the plants regularly with a high potash feed regularly to prevent this disorder. Try Chempak® High Potash Feed - Formula 4.
There are also some varieties that have greenback resistance such as Alicante and Cristal that might be worth a try next year.