I might have conquered this so feel compelled to share with everyone as I feel euphoric after many hours trying to set up my new work email address which uses GMAIL and an alias through there.
After reading many articles and discussions, and twiddling with things in gmail...
I made sure the security settings within my gmail account were switched off for the two-step verification settings, and were allowing less secure apps access (https://support.google.com/accounts/answer/6010255?hl=en). Then I added the gmail account onto my i-phone in:
1) Settings > mail, contacts, calendars > add account > other (NOT GMAIL)
2) Enter name, email address of primary account, password and a description then click 'next'
3) On the IMAP Account information page in the incoming mail server enter imap.gmail.com in the host name line, then your primary GMAIL email address and password, and in the outgoing mail server box below add smtp.gmail.com, then enter your account primary email address and password in the boxes below (don't leave them blank despite the option being there) make sure the 'use SSL' tab is on and this should add the account allowing you to choose mail and notes to be synchronised
4) To add your alias in, go back into the account from the 'mail, contacts, calendars' area, select the account, go into it via the IMAP box above mail and notes, and in the IMAP account information at the top click into the Email which should show your primary GMAIL account. In here you can choose to add another email, so add your alias and hit 'return' on the keyboard, and you should be able to tick the one you want as a default.
A suggestion which worked before I got all the above working properly was to select my other email that's set up on the phone (an outitgoes account) and in there, I could add my alias into the email line as above. This worked but only through my mobile data and not my wifi - but the above has worked and my emails are sending through my wifi.
It has all come down to disabling the two-step account verification within gmail settings (it wasn't switched on in the first place but I turned it on, had to enter various passwords into my phone, then undid it and followed the link above), and being connected to my secure home wifi (don't try in a hotel / open wifi).
Hope this helps if anyone's still stuck!