Brian Lawson's marriage index is not an official record, and he might have transcribed "Margaret" wrongly (or it could be transcribed wrongly in the Registry index he used).

I suggest you order a copy of the marriage cert from the Registry giving them the details:
Name Date Dis Vol Page Spouse
LACE, John Thomas 1919 R and 108 HERDMAN, Adelina Margaret

You can email them at: www.gov.im/registries/general/civilregistry/ under "Contacts".

If the marriage cert itself is incorrect you can send them your photocopy, and they can check the names against the original volume. If their index is incorrect compared to the marriage entry they will change it.
After you have the proof of the marriage cert., if Brian's index ref is incorrect write to the email address given in my message just below this "Will transcripts to add to Brian Lawson's index", and John will correct it.
But - if she was entered as Margaret at her marriage, which she signed, I doubt it can legally be changed.

Sue