I have deleted the Gmail account from WLM, and then logged on to it via the web and made a few changes there. In the settings, I went to the Labels tab, which I discovered refers to the folders, and selected to hide the ones that I didn't want, and unticked the box to 'Show in IMAP' for certain ones. I then set up the account again in WLM - still as IMAP. There are not as many extra folders now, but I now have an Inbox folder, underneath which (at the same level) is a [Gmail] folder, and within that (as sub-folders) are the Drafts, Sent and Trash.
I suspect it's something to do with the account being IMAP, as both my Image and Light account and Gmail account appears to show sub-folders. The Hotmail ones are fine and presumably are set up as POP. I just left them to configure automatically.
The most puzzling thing is that my Image and Light account will still not store the sent messages in its own 'Sent' folder, but continues to place the sent email in the 'Sent Items' folder in the default Storage Folders (note different names). I even tried changing the properties of the folders for that account. It had in the settings 'Sent Items', which is the name of the folder in Storage Folders, whereas the sent mail folder in the I&L account is named 'Sent'. Changing that has made no difference to where the sent mails go. Gmail is placing sent emails correctly in its own 'Sent Mail' folder.
I'm baffled, but leaving it be for now. At least it's all working - so far.