I think it is very important to keep all matters in writing so that you have a paper trial, also people are sometimes not prepared to make written statements without first checking their facts, but are prepared to give answers without checking on the telephone. I have never been divorced but I would find it almost impossible to prove this with documentary evidence.
Make sure any documents you send to the Trustees are certified copies, most solicitors will certify copies if they see the original. Send certified copies of your marriage certificate and your late husbands death certificate, which will include his cause of death, to the trustees remind them of their letter to you and if his estate was dealt with by a solicitor a certified copy of their letter or letters and ask them what further evidence they require but warn them any expense incurred will be at their cost (you may have to forgo this) as it is not your responsibility that their records are incorrect.
If they require the medical records of your late husband, as you are the spouse these should not be difficult to obtain, if all fails ask them to write to you to say you have exhausted their complaints procedure and take the matter up with the pensions ombudsman (020 7630 2200) hence the paper trial.