I have over recent days noticed this large wasp like insect attacking bumblebees and butterflies whilst they are feeding on my sedum flowers. Can anyone identify this for me.

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Unknown Insect.jpg, 27.91 kb, 240 x 231

Depending on where you are, this certainly looks like a hornet Vespa crabro and the behaviour sounds like that of the hornet. Again, it looks like a hornet but without a better picture and / or more information, perhaps hard to be definitive.