Charles Allerton & Sons, Park Works, Longton, Staffordshire, started 1859, taken over in 1912 by Cauldon Potteries but still operated as Allerton's Ltd until 1942.
Your mark seems to be one in use between 1915-1929 - the name of the pattern is often included as in yours (Willow) - I doubt you'll get a closer date than that to be honest.
Info from Encyclopaedia of British Pottery & Porcelain Marks by Geoffrey A Godden.
