Rotting of the flowers stem anywhere but especially right below the flower is typically a sign of gray mold (botrytis). This is usually the result of improper watering practices - keep water off the foliage as much as you can and only water when the top of the soil surface is dry. Gerberas originate from very dry areas of South Africa and like well drained soil and full sun.

Thanks for the reply gardengal. I'm in the UK and have since found out that they're really difficult to grow here - perhaps what you said about the african origin is the cause of this. It's a shame as they're lovely until they begin to turn. I'll have to find an alternative that can put up with our changing weather!