Boy! My article on Mercer's Troop was written a long time ago.
Since then I discovered that Mercer did get his medal and is now held at G Troop RHA's small museum
The question originally arose because the Royal MInt copy did not show him in G Troop or Beane's Troop which he succeeded to.
(although the Waterloo Muster Roll does show him in Beane's Troop).
I have since corresponded with Bob on this subject. Various regiments read the order differently; some included killed, others didn't, some included those in reserve (ie baggage guard) others didn't . In fact it is a real mess when you start delving. Gareth