Assuming that materials are standard white replacement about £3,000 prices tend to rise when coloured/effect boards and deep flow profile gutters are used but £4,500 seems a bit expensive although i have not seen job
if water is running from rear probably back gutter either not correctly dressed or tiles not correctly trimmed to it. same if front its the apron
another common point of entry is where flashings are not fixed into masonry lead should be folded a absolute minimum of 20mm - 25mm we fix with...