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Ha, didn't realise someone had just asked the same question.

Me neither.

Need to investigate the valves a bit more as one of them does adjust
the water flow but the other appears to be fake and just spins freely.

Lock shields (the one that doesn’t control the flow) often have a dummy head or a cover on them that needs to be removed to close the valve, there will be flats on the spindle, an adj spanner will do the job.

If not you’ll need to drain down the whole system 😭😖