this is more for someone who may have this problem in the future, just to keep in mind.
When I put my B17 on my old mtn bike commuter, not too long afterwards a small creak started up. It was not doing this on the other two bikes it has been on last summer and while Ive basically ignored it for a while as it wasnt bothersome, I finally tried looking at it tonight.
Had suggestions of greasing all the points of the saddle frame; also that it might be the seatpost itself being diff than the other bikes and how it holds the rails (theory here that the Brooks rails are stiffer than other seats) and/or that it is an old post (1998) blah blah blah
well, took a good listen to where teh creak was, took a gander at the seat itself, dug out the brooks wrench to see if the nut up front was the culprit, turned it a smidge (1/8 of turn) and presto, no more creak.
In fact, as soon as I put the wrench on the nut and turned it slightly both ways, the creak sound was present, so it would appear that all it needed was a slight tightening to stop the creak of the bolt up against the mount.
easy peasy and I wish I had tried it a month or more ago.