Sorry For never gettin back to this issue i didn't realise more people had replied. After conversations with the gas supplier it appears to be a rule to reduce the chance of gas leaking into the sub floor not being noticed and then igniting. the air vents were to be at least 1 meter from the recessed box. at 98 and 97 cm i didn't think the 3cm would make much of a differenc. i've now chipped away the cement leaving the 3cm of cement to stop my house from blowing up.
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