Most of the replies to this query seem to be on the right lines. I was a financial adviser until retirement 4 years ago.
We were always taught hat DISB, Death in Service Benefit was a "bolt-on" to an Occupational Pension Scheme. It was usually a multiple of annual salary. Members of the Pension Scheme were asked to nominate someone to receive this DISB, in the event of that person dying before retirement. It was usually the NOK but not always so. It could be anyone. It was left up to the Member who to nominate. The Trustees are not allowed to interfere with the Members choice, although a Pension Scheme that is managed properly would have reminded Members at regular intervals, to check that the nomination was still correct.
The important thing to remember concerning DISBs is that ::
1 DISB is non-taxable
2 The Trustees of the Pension Scheme are not "honour-bound" to go along with what the Member had first declared. They have discretion in who to pay the money too. This was because many years would normally elapse between making the DISB declaration, and its pay-out in the event of an early death. The Members circumstances may have changed and the origonal nominee may not be still alive, for instance. If the payout had not been discretionary, than how could the DISB be given to a dead person ?
In the event of a Nominated Person being dead, or unable to be traced, than the Scheme would normally approach the NOK, and proceed from there.
You are able to register a complaint to the Trustees, if you think that the payment to The Other Woman was wrong, but it is unlikely that they will change anything, unless there is just cause.