Forgot to inform HMRC company is trading

Can anyone help? I set up a company and didn't inform HMRC before the 3 months was up. The company was set up just to receive a one off payment with no intention of trading. I have a UTR number but when I try and proceed online it doesn't seem to recognise this presumably because notice to file tax was missed. There doesn't seem to be any way of accessing help or finding anyone to speak to. I'd prefer to pay the tax rather than evade it. Can anyone offer any suggestions? How do you speak to someone at HMRC? How do you announce you will be filing if it is past 3 months? Any help appreciated, thanks

Hi,
You will need to contact our self assesment helpline on 0300 200 3310 or contact our webchat facility at Contact HMRC, quoting your UTR number and advise them of the payment.  
They can arrange for a tax return to be issued.  
If you have tax to pay, then you can pay the tax in various ways, as advised at Pay your Self Assessment tax bill.  
You should add the letter 'K' to the end of your UTR when making any payments to self assessment.
Thank you.