A warning to any other owners of this router.
Have upgraded the firmware on my previously functional Linksys RVS4000 router
detailed here to firmware version 1.1.09, from Linksys' site only to find it has a bug. The bug stops it recognising a cable modem on the WAN port and thus never makes a connection to it. Mine is now well and truly 'bricked'. I will give the rescue utility (also from Linksys' site) a try to see if I can get a working version on there; sadly the original firmware does not seem to be available anywhere to downgrade it - despite many other customers requesting it on the support forum at Linksys.com. Why they keep this version available for download while it is completely FUBAR'ed is beyond my comprehension.
So to summarise, be very careful about what you upgrade this router with, but I suppose best of all, is to make sure you've got a good enough back-up (if that is indeed possible) of the firmware before you start upgrading. Or better yet, if yours is working satisfactorily and not broken, don't fix it
Ever learning through mistakes it seems I am.