Hello,
I've recently encountered an interesting passage in the Personal memoirs and correspondence of Colonel Charles Shaw (vol I, 1837) concerning a military musician who later achieved rather high rank in the Brunswickers during the Waterloo campaign.

p.48 "While riding one day with two field officers of the 90th regiment, we were accosted by a Brunswick general officer on horseback with his two orderlies behind him. He addressed Colonel Wright, who did not know him, familiarly and by name. It was General Offermann, at that time commanding the corps of Brunswickers with the army. He had when a boy enlisted in the 90th; and, having a taste for music, had risen to be master of the band. When Minorca was taken [in 1798], there were two German regiments in garrison in the service of Spain, who volunteered into the British service. The 90th were then in garrison in Gibraltar. Few of the officers spoke German, and Offermann, being a very smart, active fellow, was sent as serjeant major, and at length became adjutant of the 97th, (the "Queen's Germans.") His conduct in this capacity was so correct, that on his return to England, the Duke of Brunswick appointed him his aid-de-camp; and when the Duke was killed at Quatre Bras, he succeeded to the command, and gave universal satisfaction."

I have attempted to discover more about Offerman(n). A book published in 1852 also states the Brunswick corps was commanded by a Colonel Offerman after the Duke's death at QB. https://books.google.co.uk/books?id=8mQDAAAAQAAJ&pg=PA276&dq=Offerman+brunswick+waterloo&hl=en&sa=X&ved=0ahUKEwim5Yz43f7kAhVRRhUIHT3NB4EQ6AEINDAC#v=onepage&q=Offerman%20brunswick%20waterloo&f=false
Also, the London Gazette states that an "Adjutant Ernest Offermann" received an appointment as ensign in Minorca Regiment, dated 2 August 1800. Surely this is the same fellow.
Shaw's story seems credible but I'd ideally like to know more about Offermann - can anyone share further information or point me to relevant sources?