Thanks for all your help.
Although I didn't photograph the document at the archives the description of the item says "Papers from administration of the estate of Grace Ellen Frederica Blandy decd, late of 40 Greycoat Gardens, spinster who died 12th Nov 1918. Capt Mark Kerr of Fairy Cross, Seaford, Sussex, a cousin of the decd and a trustee of her will (probate 24 Jan 1919) was responsible for payment of an annuity to Lucy Harby which reverted to the residual estate upom death of Lucy Harby on 21 Oct 1924."
So if he is a first cousin I guess I have to go through all female siblings of Graces parents and find one who married a Kerr. Does that sound right?
Thanks again.