Was Brown's PMQ's quotation actually made, or just made up?

I never thought that BBC's "The One" show would be involved in a political discussion, but presenter Adrain Chiles has unwittingly just dropped Gordon Brown right in it.

In this afternoon's PMQ's, Gordon Brown was being questioned about the failure of the FSA to act during the Northern Rock crisis. Gordon said the FSA was doing a fantastic job, even though the FSA themselves had admitted that it failed to regulate troubled bank Northern Rock adequately. During the exchange Gordon attempted to start throwing a few quotes back at David Cameron. One of those quotes was from Frederick Forsythe, who Gordon claimed had said about Cameron that he had "no basic grasp of arithmetic".

However, when Frederick Forsythe was shown the footage of Gordon brown quoting him he stated the he had no idea when he made that particular quote, or even if he had what context it was made. In fact, he seemed a tad incensed that he was being misquoted.

Update: Apparently, the quote was taken from a letter that he had written to the Times in 2006. I haven't read or seen the letter, so can't tell what context it was written in.