Data for the United Kingdom (UK) Manchester Butterfly Atlas produced conflicting species loss rates for increased urban development. In particular, a very low rate of loss was recorded (0.19 species for every 10% increase in urban cover) for the Mersey Valley mapped at a high resolution of 1 ha units. It was suggested that sampling artefacts (uneven recording) or failure to distinguish vagrant individuals from breeding populations cause this. Herein, results are reported for 30 sample squares, within the Mersey Valley, surveyed uniformly throughout 1999. It is shown that loss rates are as high as areas mapped at lower resolution over wider areas (0.67–0.68 species for every 10% increase in urban cover) and that increasingly stringent definitions of urban cover result in higher loss rates. Comparison with the data from the Atlas, but for the same 30 sample squares, indicate that the low rates at a fine scale for the complete Atlas data are more likely to be caused by uneven recording than from failure to record species status. However, progressive sampling of squares, despite uniform recording, will inevitably cause a reduction in loss rates of total species for increases in urban development.