Thanks for the reply - after many weeks of blaming the router I have found the issue, (which I will detail here for others):-
It is not a router setup problem - even through my last router worked perfectly!!
The issue is with windows - when you change anything to do with the wireless, (security, ssid, type etc), windows XP, Vista and 7 automatically changes everything to "Public" network security level. This means that machines that previously shared files will no longer do so. Please feel free to thank microsoft for this excellent example of "nannyware". Once I finally, (doh!), realised it was not the DIR-655 it only took a few minutes to re-establish the home group and shared folders.
Hope this helps others....