Hi there
Your William Emmott (or Emmett on the Waterloo Roll) was indeed in the Royal Horse Guards but was a Corporal not a Captain (quite a difference!). He was however, commisioned in 1828 - a very rare feat - as Quartermaster of the Regiment. He was placed on the half-pay lists in 1831.
If you have any more info about him I'd be very interested to hear it.
Kind regards
Martin Aaron