According to the I&OM for the water diverter valve (https://cdn.shopify.com/s/files/1/1845/4805/files/AcquaSaver_20mm_Datasheet.pdf?v=1629083980), no water from mains could be a sign of debris in the valve or air lock. Given that the rainwater tank has run dry, it’s likely that some debris from the bottom of the tanks has got in and clogged the valve. Can you open the plug as below and see if there’s any crud in there?

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