Shome mishtake, shurely?

[88V8]

I had an AVC with ... a well-known provider.
For the last five years prior to retirement, no bonuses.
Sign of the times, I thought. Shrug.

But now, out of the blue, and five years after I retired and took the AVC as a lump-sum, they sent me a cheque for about 6% of the pot.
Apparently there had been an error. And they paid me interest on the underpayment, and paid the tax I might have owed.

All jolly decent of them.

Just curious, is there any way that one can know whether or not a final redemption value has been correctly calculated?

V8