They have been still standing tall but with bowed heads and died flowers for some time. At the weekend we decided it was time for them to come down and so we saved the seeds from one head and the others put onto freecycle (and had someone wanting them straight away). We could have had saved them for feeding the birds but next door feeds the birds already and the mess that it creates is a constant problem for us!
Meanwhile, the smaller multi-headed sunflowers in the front are still doing well. Certainly a good idea I think.