It is height adjustable but fiddly to do. Under the float chamber on the stem is a small grey clip. remove this and the valve body can be rotated slightly and then moved up or down. Once set, rotate back and replace clip. The final water level will be at the top of the float chamber. Your valve looks to be hard against the back of the cistern so may not rotate to adjust (can't remember which way they turn). If that's the case, remove whole valve from cistern, adjust it and then replace it. Takes longer but less fiddly.
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