I recently had a small surprise, as I had a reasonable tax rebate. I did not initiate the rebate, thus was surprised when it came through.
I am still in employment, i.e. not early retired, so my tax affairs are simple. But, as I have moved from a reasonably paid job to a lower income, my tax codes did not keep pace with the change in circumstances.
I believe there are 'paid for' tax checking services but I would recommend the following link.
This give simple advice on how to check your tax code.
Another tax issue if you have children is tax credits.
I urge everyone to check their annual statement as mine was wrong and I had to call to have it amended, or lose any benefit
Similarly, for a child in higher education, child benefit is still payable until the child reaches 18.
So,I have had to fight to retain these benefits but have not missed a beat since my first child was born.
If you are entitled to it, then claim it. If you don't ask, you don't get.
Keep on Saving.