The tradition is mentioned here
http://www.british-history.ac.uk/vch/worcs/vol3/pp561-566
with a source.
But from the dates - Henry IV / Glendower - the Lady Washbourne involved wasn't Joan Mitton, it must have been her husband's grandmother, the heiress Margery Poer.
Wichenford Court is on the market at £1.7m, ghosts included
http://www.countrylife.co.uk/country-life-root-category/property-news/wonderful-worcestershire-properties-sale-67076