UK Tax Rebate, and Paying NIC's
 

Hi all, I've been a USPR since Jan 2019, and before that I was on an L1 visa from Aug 2017. Before that I traveled back and forward between UK and US for about 18 months.
It became clear that I was due to pay US tax, and the company paid for a tax attorney to review things. They ascertained that I needed to pay US tax as far back as 2016 and submitted a tax return to the IRS on my behalf for 2016 and 2017, which was accepted and paid. I filed 2018 taxes jointly with my US wife.
I got a partial tax return from HMRC for 2017 but I think may be due some back for 2016 too. Any advice on how I go about claiming this?

Also how do I start making voluntary NIC's?


Thanks all.