Just a thought, LG:
If you've not already got a Government Gateway account, apply NOW for one. (It won't matter if you've not yet got your figures available for your tax return). That's because the security system means that you have to wait for an access code to be POSTED to you.
So you can't wait until the very end of January (which is the deadline for online tax returns) before applying for a Government Gateway account, as there would be insufficient time for the postal part of the process to happen. (You only have to go through the process once. In future years you can log straight into the HMRC website).
https://www.gov.uk/government-gateway
(PS: If you've already got a Government Gateway account for something else, such as if you've previously claimed state benefits, you might find that it doesn't work with the HMRC website. I couldn't get in using a Government Gateway account which was originally set up to access the DWP's services. When I phoned up to get some help I was told that, even though it's meant to be a universal system, some Government Gateway accounts won't work with the HMRC site. I was told that I had to get a second Government Gateway account).