Many thanks for your input. Brute Force and ignorance on my part prevailed and fan was removed. Problem was a missing key lock, which prevented the fan being tightened. once replaced by local trades man the fan was tightened and "silent running" resumed.
Thanks.
Fan was removed by holding the drum, using two screwdrivers to behind the fan to lock it. enabling the retaining bolt to be removed. the missing keyway was cleaned and replaced and the fan replaced and tightened.
John
April 2008