I have read the article Tom & Gadget and dont think he was demoted. see below
Brudenell was disliked by the officers under his command because of the way he had used the purchase system; his temper also caused perpetual quarrels. In 1833, he illegally ordered one of his officers, Captain Wathen, into custody at Cork. Wathen defended himself so well at a court-martial that Brudenell was persuaded to resign the command of the 15th Hussars.
he left the army in 1852 and is living in 1861 on his pension
and this is probably his death
Deaths Dec 1866 (>99%)
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Wathen John 61 Brentford 3a 43
on freebmd