Andreas, I don't know if these will help, but they might be the place to start looking at any rate:
Simms, Brendan '"An Odd Question Enough": Charles James Fox, the Crown and British Policy during the Hanoverian Crisis of 1806' Historical Journal vol 38 Sept. 1995 p 567-96
Dwyer, Philip 'Two Definitions of Neutrality: Prussia, the European States-System, and the French Invasion of Hanover in 1803' International History Review vol 19 no 3 August 1997 p 522-40.
Dwyer, Philip 'Prussia and the Armed Neutrality: the Invasion of Hanover in 1801' International History Review vol 15 1993 p 661-683.
I think that there is also a good deal on Prussian policy in The Impact of Napoleon. Prussian High Politics, Foreign Policy and the Crisis of the Executive, 1797-1806 (Cambridge University Press, 1997)
Hope this helps.