Sorry Uwe, I forgot to add that the geometry package allows me to set up exactly what I need in the way of margins. It can set: paperwidth = inner + width + outer [I am calling them inner and outer rather than left and right, because it is double sided] where inner and outer are true margins, not used for printing. and width = textwidth + (marginparsep + marginparwidth)
It is the outer edge of this (marginparsep + marginparwidth) region which I attempted to justify my figures to. My requirement was that the outer edge of a figure line up with the outer edge of the marginpara, and the figure could extend back as far as needed into the text area, with any space left available for caption. I was having difficulty with this and I noticed that memoir by itself could perform many (though not all) of geometry actions. Then I was overjoyed to get your suggestion of using "overhang" but subsequently "underjoyed" to find I could not free it from wrap. If I cannot free "overhang" from wrap, I shall go back to wrestling with geometry. Thank you again for your help. Cheers, George