Mares tail is one of those weeds that you just can't dig out. The 'roots'
can go down as far as several meters deep (have heard of 10m or 30ft and can
sread along about the same distance. The only way is several applications of
glyphosate. It will keep coming and coming but if you are lucky in 5 or 6
years you may get rid if your neighbours get rid of it in their gardens.
You do need a wetting agent, eg fairy liquid (at your own risk) as it can
just run off and not be absorbed. Most roundup type weedkillers contain a
wetting agent.