Did hard reset of WRT54GS v.5 after firmware upgrade to v.1.50.8 then full power cycle for modem, router and pc.
Tested the speed by downloading SP2 whilst running a trial version of Network Magic Premium with Speed Meter Pro that allowed me to monitor the WAN and LAN speeds.
Got a throughput of 9.98mbps on the WAN according to Speed Meter Pro. The Windows download information settled on 1.16MB/s. All wired LAN connections are 100Mbps and the wireless connection is 54Mbps.
Using Speakeasy speed tests to NY concurrently on two wired desktops gives Download speeds of 6+Mbps and Upload speeds of 470+kbps on both machines.
Looks like this has settled the issue of my 10meg connection being effectively routed through the WRT54GS.
I can recommend Network Magic for checking and managing your home network. There's a 30 day trial of the Premium edition which drops to a Basic freebie afterwards. I have reservations about the purchase of the Premium version being on an annual subscription basis, however it's not that expensive considering the ease of use of the software and it's GUI. I'll see what the freebie leaves me with after the trial period.
Details here
http://www.networkmagic.com/
Thanks for all the helpful advice and I hope that the comments I've added will assist others in getting the best out of this router. From my reading I know it's a mixed bag of tricks and that there's a certain love / hate relationship with the later models like mine.
Wonder if I'll be attracted to 3rd party firmware? For the moment though .... if it ain't broke .....