Thank you Ron and Oliver for your very helpful replies. The sources you have shared are invaluable, and I've been able to find even more now that I'm aware of the correct spelling (Olfermann not Offerman).

In case other group members are interested, Olfermann is discussed at length in John Montmorency Tucker, Tales of the Camp and Cabinet (London, 1844), Vol II
See here: https://books.google.co.uk/books?id=SGI1Gk7hclEC&pg=PA8&lpg=PA8&dq=Tucker,+Tales+of+the+Camp+and+Cabinet+olfermann&source=bl&ots=19IsVd4WK0&sig=ACfU3U36ppt0OTVLGv7qx5lIK-zMFe8Wvw&hl=en&sa=X&ved=2ahUKEwjGwtSMm4LlAhWGT8AKHRqADwgQ6AEwCnoECAgQAQ#v=onepage&q=Tucker%2C%20Tales%20of%20the%20Camp%20and%20Cabinet%20olfermann&f=false

The story in Tucker may well have grown in the telling, but the basic details (born the son of a Brunswick tailor, became 90th bandmaster etc) correspond with other sources.
As an aside, I find the inconsistencies in Olfermann's first name rather peculiar - he's variously Johann Elias, Ernest and Max!