Leigh,
The inter family agreement means the Lord Great Chamberlain in the next reign will be the present Marquess of Cholmondeley or if he does not survive till then,his son.
When Prince George succeeds to the throne the Lord Great Chamberlain will pass to the descendants of the last Earl of Ancaster and the LGC will be Sebastian St Maur Miller(b1965) or more likely,his son Merlin who was born in the 1990's.
When that happens Merlin will no doubt be Lord Willoughby d'Eresby that peerage having been called out of abeyance.Abeyance will occur on the death of the present Lady Willoughby d'Eresby and the co heirs are the children of her 2 aunts,daughters of the 2nd Earl of Ancaster.Sebastian Miller is the senior co heir,so more likely to have the abeyance terminated in his favour.