Plusnet hasn't got anything to do with it, David.
While Incredimail might well have configured the correct settings for POP3 or IMAP, that won't help at all if neither POP3 not IMAP are actually 'switched on' on Google's server. As I suggested before, log into the Gmail acount (via the web interface) and check that one or the other is actually enabled there.
Prudie:
Microsoft have a nasty habit of using the same names for two entirely different things, which leads to all sorts of confusion.
'Windows Live Mail' is a computer program which enables people to access their email accounts without going via a web page. (It's similar to the program called 'Outlook', which is used in almost every office in the country). Microsoft now longer make Windows Live Mail available for download (although it will still work for people who've already got it).
Microsoft also (stupidly) used the 'Outlook' name to refer to the web-based email service which you use. To confuse matters even further, they then tied the word 'Live' to it. However it's still got absolutely nothing to do with either the program called 'Outlook' or the one called 'Windows Live Mail'.
You're using the web-based email service provided by Microsoft. David's mother is using the email service provided by Google (Gmail). She can access it via a web page but she prefers to use a dedicated program to do so. She previously used a program from Microsoft (Windows Live Mail) but is now forced to use something else.