Akin Kaki

I worked on a project for second employer for a week and got paid (468£ taxed at 20%, at the rate of 27k per annum) this is added as second employment on my HMRC record. However this is only one time job and won't working on it anymore. My first job already pays me 27k per annum (permanent job) taxed at 20%. Now I got a message from HMRC saying my second job needs to be taxed at 40% and I have underpaid 100£ for this tax year. My question is, the second employment is only one time job and I won't be receiving anymore payments, so my annual income for this tax year will still be under 28k, so should I still pay the 100£? Or, will it be removed after calculating total annual income at the end of year?. Also will this second employment be removed from my records?( Maybe removed after not receiving any pay for few months?) Or do I need to do anything from my side to remove it? Thanks