“The introduction of non-A1 uses into the District Centres will not be permitted if they would have a detrimental effect on the vitality or viability, or character and function of the centre or any part of the centre. Examples of detrimental effects include: creating a concentration of non-A1 uses, weakening a concentration of specialist shopping; weakening local convenience shopping; creating dead frontage; reducing the attraction of the centre to shoppers; threatening the viability of individual shops by effectively isolating them from other shopping facilities; and introducing unacceptably high levels of activity. The balance of A1 uses and non-retail uses should be such as to maintain the attractiveness of the centre to both shoppers and retailers.”