OK I've managed to fix this myself - so I thought I'd share my solution, in case anyone has a similar problem...
Create a new (temporary) gmail label
Working from within GMail's web access - select all of the missing emails TOGETHER WITH the more recent emails (I'll explain why in a minute).
Assign these emails to a newly created label
Archive those emails (Which will make them disappear from GMail's Inbox folder - as that's all that Archive does)
Go to the Label View/Folder for AAA and select all of those emails
Then "Move To... Inbox"
The emails will now show both the "Inbox" and "AAA" labels
With all of those emails still selected... Under "Labels" untick the "AAA" label and click apply
Check in GMail inbox and your mails will be there. Check too on your iOS device and the mails will have reappeared too.
Now back to the question why you have to select the more recent mails which you could already see on your iOS device.... It would appears that iOS Mail sorts the mails on the DATE UPDATED and not the DATE RECEIVED, so if you hadn't select those additional mails, then they would have appeared to have disappeared (in fact just shunted to "Older Mails" even though they had been received more recently)