In the UK, alien grey squirrels have been replacing the native red squirrel for >100 years and by the turn of the century the distribution of grey squirrels had extended to most of central and southern England and Wales (apart from some islands), and large parts of Scotland and Ireland. To understand how this species replacement may continue into the future, we explore 20 years of squirrel records (1991-2010) collected in an unsystematic way by volunteers in northern England and Scotland. To aid analysis, we standardised the data to one record of squirrel presence per 4 km square per year in each of six geographic regions, two in northern England and four in Scotland. Over the 20 years and across all regions, the number of 4 km squares that returned the presence of squirrels increased from 23% to 80%. The evolving distribution pattern of red and grey squirrels differed among the regions and was complex with occupancy records for many 4 km squares changing from red to grey squirrel from one year to the next, but also some changing from grey to red squirrel. The net rate of change, however, was from red to grey squirrel with the exception of the more recent years, 2007-2010, when this trend became less distinct. Estimates of the complete loss of red squirrels assuming current trends, varied from the 2020s in northern England to 2104 in North East Scotland. The recent incertitudes in the replacement of red by grey squirrels may, at least in part, result from increasing efforts to control grey squirrels in these regions. Although the volunteer data presented are valuable, monitoring aims must be clearly defined and sampling designs involving volunteers and professionals must be systematic and of sufficient effort to obtain reliable information on changes in distribution and population trends.