Just a quick thankyou to Bill, Peter & Micheal Peters for some invaluable 
feedback on my options for handling DC Date meta data. It seems I have 
several possible paths that I can follow, and I will need to evaluate them 
further.

One question that was raised was why I needed to use swishlastmodified. Only 
because I am using a set of search indexes, some of which are generated 
directly from the filesystem and not via spidering. Those automatically have 
the swish lastmodified property set, but may not have reliable 
DC.date.modified metadata. I thought it would be cleaner to have the valid 
last modified metadata stored in the same property in all indexes , so that I 
didnt have to hack the search cgi to use either swishlastmodified and/or 
'DCDateModified' when presenting the last modification date stamp in the 
results. 
To take a different tack perhaps I could reconfigure the file-system based 
indexes to make swishlastmodified as alias for 'DCDateModified' and then 
modify the result template to report just 'DCDateModified' ...

Peter Farmer