Am I entitled to a tax refund?

I started a new job but did not receive a p45 straight away due to being owed overtime from my previous job and them paying that in the next pay month. So for one months pay it has shown that I was employed by two companies and therefore have paid quite a large amount of extra tax but I wasn’t technically working for my previous company, just waiting for over time money I was owed. Am I entitled to get this money back? I had to do a self assessment to pay back overpayment of child benefit and did leave a comment in the section where you could let them know if you think your owed anything. But I have had a letter through just stating the amount of money that I Owe for the child benefit, no sign of a refund of that extra tax I have paid. Thanks

Hi
Please refer here Tax overpayments and underpayments