Francis from the Freetrade team assisted and solved the mystery! 
This deposit was the cash value of the remainder of a fraction of a share after the Investec/Ninety One demerger where you would receive 0.13751 Ninety One Shares for each Investec share you have. So after I received a few whole Ninety One shares there was a remaining 0.08787 of a share left and that was worth 19p.
All is good now!