Thanks for the comments.
ddk - interesting comment ref sharpening. Aperture is left to automatically adjust the 'raw fine tuning' - all I've adjusted is the contrast, albeit this includes 'Definition' which enhances edge contrast. Has a similar effect I suppose. I wonder whether this is exacerbated by your monitor - mine's not to sharp, maybe causing me to 'oversharpen'. I'll check appearance at work tomorrow (work CRT monitor is rather better than my old iMac LCD screen.
I've not added any 'Definition' to this one - better?
Edit to add - I've just checked the file for the old stone cross photo - no definition added, just upped the saturation & contrast, then converted to monochrome, red filter applied, and that's it.