just to update on this for anyone else with this issue...I replaced the piezo as planned but this didnt fix it...I could still hear the piezo very quietly but during remounting it I twisted the motherboard a little and it came good for a few seconds before going silent again...I tried to simulate this again and again but no joy..re-tightened and tested all connections but absolutely no luck so in the end I swapped it for exactly the same motherboard from the replacement unit Alert Electrical had sent me and it worked fine...swapping the connections back againto the old unit it stopped working...so there you have it folks a faulty main board on the bell box..
All working now Thanks everyone for the help in solving this one
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