What was the flower?

The plant is Gomphocarpus physocarpus - synonym Asclepias physocarpa - common names: balloon plant, swan plant, hairy balls, family jewels tree - a species of milkweed. The plants main claim to fame is playing host to the Monarch Butterfly.

lavae/caterpillar via               butterfly via

Monarch butterflies cannot survive without milkweed; their caterpillars only eat milkweed plants so the butterflies need them to lay their eggs. The leaves of milkweed are poisonous to most animals as they contain chemicals called cardenolides which does not affect Monarch caterpillars. They store the chemicals in their bodies which makes them poisonous to other animals. When the lavae turn into butterflies the cardenolides remain, protecting them too.  

It can grow up to 2 m tall - a slender perennial erect shrub.

It has these extraordinarily large distinctive prickly pods - I wondered what was inside

Until I saw this split pod packed full of seeds. Each seed wore a luxurious silky white flowing parachute
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Whilst doing some research I discovered that there are over 100 species of milkweed and all of them are quite different from one another. Some have bright orange flowers in clusters, others come in shades of white through to yellow, pink, purple and red. Only Gomphocarpus physocarpus and a sub species Gomphocarpus fruticosus appear to have these large round hairy seed pods. However, most milkweeds do have extraordinary seed pods. It is worth taking a look at them on google images here.
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Congratulations and well done to Celia Hart and Rod Lutes who both identified the flower correctly.