I also have a Margaret who was always called Bunty as a child, however, if you look for births of Buntys in FreeBMD, you will find that the first birth registered with that name was not until 1912. Between then and 1960 there appear to have been around 25 Buntys and one Buntie registered in England & Wales. Could it be a Scottish name in origin, perhaps?
Gillg