Can you help me to identify the yellow and black insect to the right of these pictures? it is enjoying the feast of sap on my plum tree (Herts, UK)

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It's some sort of sawfly, which is a wasp-like insect, only distantly related to true flies (like the black one shown nearby). I'm sorry, there's about 500 species in Uk and i'm not familiar with them, this site looks quite useful (once more data) and can submit observations : http://www.britishsawflies.org.uk/about.html