Just an update re this issue. Cooper Tyres wholesaler have been fantastic and after plenty of tests, etc, they have swapped the tyres (that had done about 15,000kms) with brand new Mickey Thompson ATZ, and the harmonic vibration has gone away! Also makes me realise how much road noise the ST's made! Peace and sanity returned! (Just waiting to replace front steering arms and bushes, as these are worn, plus was alerted to the brake recall after nearly remodelling my garage!). Thanks to all......