Peter
I think there is too much backlight at the top. It makes it hard to get the details in the flower heads. But I know how difficult it is to get shots such as this right. I did a similar shot last year and I used a 5 inch high band of black art paper behind the flowers to shield the brightest light, but I still had plenty of light on top. I often carry it with me in the spring/summer as it forms a useful backdrop for flowers. I also find one of those collapsible reflectors useful - mine has a mesh screen which is good for diffusing the strong overhead sunlight, which I think you could have done with here.
My shot is here on my website if you want a look (second row) The Lysimachia (first shot) and the Primula Viali were also taken outdoors with the black backdrop and overhead diffuser. http://www.carregwenimages.com/section135630_60063.html
Keep meaning to visit Wisley, as the wife is an RHS member. Worth a visit?
Agree with Rob. The center flower is nice and the surrounding flowers frame it nicely but the background is distracting.