Very late now and will not be much use to you Sue but this may help you in the future or others searching... I suggest you look on the postcode plants databasehttp://www.nhm.ac.uk/nature-online/life/plants-fungi/postcode-plants/
and check it grows in your area, then as Sarah suggests armed with a photo and identification details go and in search. Failing that you can buy a Yarrow plant from most herb nurseries, or look online for suppliers such as Baldwin's and buy they dried herb and make a tisane with it and when allowed to cool so it's just warm use it to sluice the mouth over the tooth. I've had good results with a warm tisane of Lemon Balm for toothache in the past and swear by it.