Don't leave it too late to register.. you need at least 7 days before the deadline...

If you are submitting your return online for the first time then you need to register online with HMRC and that takes time. 
You will need your 10 digit unique taxpayer reference number (UTR) which you will find on most letters from HMRC. 
Once you’ve enrolled you will be sent an activation code in the post which SHOULD arrive within 7 days.
 You can then use the code to activate the service and then finally submit your return.
If you have left it too late then we can submit on your behalf without you having to register.