Finally fixed the laptop. After some prevarication and testing I decided the wireless card might be faulty so managed to force myself to get out and buy a new one.
This time instead of the Netgear WG511T which worked well for several years, I bought a D-Link DWA-610 ‘Wireless G Notebook Adapter’. Installed it, connected it, and it works.
So far it is *nowhere* near the speed of the old card but that might be a function of it’s ‘iffy’ connection with the Belkin router I have. Having said that, the router itself is nearing the end of its use-by date and is showing its age badly, needing constant reboots to keep it functioning at all!
Maybe a firmware update would solve the problem, but it really is ‘old tech’ compared to the new breed of machine available. It might be simpler all round to just replace it… if the cost is acceptable that is.