Mary was a Plantagenet Princess of England, given that she was 6th in direct male-line descent from King Edward III of England]. Accordingly, since the illegitimate children of John of Gaunt, 1st Earl of Lancaster] were later legitimized, their male-line descendants should hold the title of Prince/Princess of England, or very least, of The House of Plantagenet. Given that, Mary SOmerset should be known as Princess of Somerset, as the current Eugenie and Beatrice are Princesses of York, and are not known "of England."