How do I link my married name to my NI number?

I'm trying - in vain - to set up a small business. However, I am getting completely stuck at the ID stage of Self Assessment. The problem appears to be that historically I was self-employed. This ended 4 years ago. I then got remarried. My old account won't let me change my name because it doesn't recognise my new name on my passport. I've then created a new email address and got a new gov gateway code - only to stumble because it won't accept my NI number - which I assume is linked to my previous name! This has cost me so much time! All I want to do is change my name!

Hi Amy,
Can I suggest you contact the NI helpline on 0300 200 3500. Lines are open 8am – 6pm Monday to Friday?
They should be able to make any necessary updates to your personal details during the call.
Thank you.