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Thanks for the detailed reply, Chris.
Thankfully, when Mrs NJ turned her machine back on this morning, lo and behold she was able to log in OK. These mysterious things aways intrigue me. The machine had not been touched since early afternoon. Went out and returned about 9pm to face the problem.
I did notice the "forget" option when I was fannying about last night, though I didn't try it. I'll keep your instructions handy for the next time it falls over!
Many thanks again.