You need to register as self employed (I know you've a main employer but it is the same situation as me) on the HMRC website. Describe yourself as a sole trader. It takes all of 5 minutes. Then, when they come bakc to you I would be honest with them, say you got the wrong end of the stick, give them your data so far and ask what they can do to help you out.
Being as proactive as possible might help convince them you've goofed as opposed to are deliberately avoiding them.
