That kenwood is a UK model so if your in the US it's tough to source. The ring you need should have an outside diameter of 76 mm and an inside diamete of 56 mm.
This is a uk ebay link with the exact part.
1 Kenwood BL300 Blender Liquidiser Rubber Seal Gasket
This is a us amazon link with something that might work but not exact. There is a uk amazon equivalent if that works better for you.
Amazon com kenwood blender seal
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