900 vs 1800
Andy Watt's absolutely correct. This is all down to physics; 900 has a longer wavelength than 1800 so carries further and penetrates better. I used to work for the magenta people and the techies there deluded themselves by constantly 'drive testing'. Get real! Normal people, without pointy heads, want in-building coverage (especially for mobile broadband which is the great hope for the networks' declining voice revenues) and 1800 just doesn't cut it.
With 1800 you simply need more masts to get the same coverage & quality than you do with 900 so highlands, islands and indoors suffer.