National Insurance Refunds
Often you can be employed and self-employed in the same tax year. Unless deferment can be put in place it is necessary to reclaim any overpaid national insurance contributions at the end of the year.
In theory those which simply arise because you had more than one employment in the year and have paid more than the maximum class 1 at the full rate should be spotted by HMRC but this cannot be guaranteed.
With self-employment and employment though this is unlikely to be noticed by HMRC. It is most likely to be significant when you ceased self-employment during a tax year and then became employed.
We have just obtained a refund of over £2,400 for one year for one client which was missed by one of our national competitors !