Thanks Neil - and Thanks to MrWilson and Ken,
So, if I understand all of this correctly:
The 11th Foot Regiment DID exist in 1808, so it is quite possible that my ancestor was part of that Regiment at this time. The link you provided to the National Archives site has a record of John Hall joining at this date, serving in the 11th Foot Regiment and being discharged in 1836.
The 11th Foot Regiment becomes the Devonshire Regiment in 1814 - and this is NOT a part of the Grenadier Guards. It is something quite separate, though it would have its own Grenadier company.
The Grenadier Guards used to be called the Royal Regiment of Foot Guards until 1815. This Royal Regiment of Foot Guards is NOT the same thing/part of the 11th Foot Regiment (later Devonshire Regiment)
I had originally misunderstood some of the earlier posts (my fault - sorry) and I thought that the 11th became part of the Grenadier Guards.
So, it appears that it is possible that:
1. John Hall remained with the 11th and may have been part of their Grendier company. Though the fact that he doesn't show up on the list Ken provided is a bit of a snag.
2. John left the 11th and joined the Grenadier Guards before 1815.
3. John moved to another group altogether and was part of their Grenadier company??
Great - I have some wonderful leads to explore. The links you have given me will be a great help.
I have researched ancestors who fought in both world wars, but I have never tried to research anyone's military service before that. Thank you all so much for your help. This was completely new to me and you have been wonderful. I am very interested to find out about John, my 4x great grandfather!!
RK