Remove door trim, unclip electric plug to the motor drive assembly and using a multimeter check for power when operating the switch. If you have power, then remove the motor to test it by itself.
If the motor seems ok, then it will be the window mechanism at fault. They're a bit cheap and don't last forever. A new one is about £ 70 off ebay and is easy enough to fit. Upon reassembly, allow the window to travel fully up and down before tightening the two window clamps up.
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