Request.Querystring And UrlDecoding?

(asp.net 2.0, c#) I have a page requesting a qyerystring looking like this ?test=b%E4st, %E4 is the url-encoded letter ä. I can't change how this looks and encodes since I have no access to the page doing the request.

Problem: string strTest = Request.Querystring["test"].ToString();

If I then, for instance, just Response.Write(strTest) the character ä (%E4) is broken. Displayed as a question mark. I have in my web.config the requestEncoding and responseEncoding set to utf-8. That is the way I need to have it, and I can't change that.

I've looked around for a solutions for this and the issue seems to be iso-8859-1 vs utf-8 in the querystring and url-decoding. It seems the Request.Querystring automatically url-decodes the string using the default encoding, in my case utf-8. I need it to url-decode using iso-8859-1 instead. I can achieve this by altering the web.config, but as I said earlier this is not an option.

I have tried to do this:

Encoding enc = Encoding.GetEncoding(28591);
string strTest= Request.QueryString["test"];
strTest= HttpUtility.UrlDecode(strTest, enc);

Not working, since the Request.QueryString already has url-decoded the value using utf-8.