This time round, I needed to put a load of diagrams on one page, and wanted them to appear in specific places, rather than have to refer the reader to "Figure n" or whatever.
In tex.stackexchange.com/questions/19766/how-to-control-the-position-of-floating-images I found a nice solution: \FloatBarrier. You need to use the placeins package, and write \FloatBarrier wherever you want to pin down the position of a float.
Trouble was, the last picture was falling off the bottom of the page, even though there was plenty of room. The cause of this turned out to be the totalnumber parameter, which was set to 4 (so the fifth picture was ending up on page 2). Changing this to a higher number solved the issue. Here is a good description of how the parameters work: