The above book mentions the Lithuanian prince that was baptised in London and gives a date of 1364. It refers to the 11th Earl of Warwick as being the sponsor of the Lithuanian Prince. It also mentions that Thomas Beauchamp (the soon to be 12th Earl of Warwick) went back to Lithuania in 1367 so the young Prince may have travelled back with Thomas Beauchamp. The 11th Earl of Warwick died in 1369.
Quite a bit of interest in this family at the moment as the BBC are showing a series called "The White Queen". The Earl of Warwick and his daughters feature in the series Anne Neville being married to Richard 111. Richard 111's body was recently found in a car park and has yet to be buried.
Thomas Beauchamp 12th Earl of Warwick was great grandfather of Queen Anne Neville. Richard 111 is to be buried in Leicester but some think that he should be buried in York and some think that he should be buried with Anne Neville in London.