The problem may not be with failing to register the marriage, but with the marriage indexes being hopelessly inadequate. There has been some research done which highlights the fact that enormous numbers of register entries have actually not made it into the indexes. I'm not quite sure how you go about finding one of these missing from the index, but if you had a fairly close idea of when the event took place you could try getting the appropriate registry office to do a search for you (rather than going direct to the central Family Records Office).
The other option is to get creative about spelling and search the indexes under various mis-spellings. Think about what the name might look like handwritten, and go for any variant that may possibly be confused. For example, I have SENNETT that has been spelled with the more obvious vowel changes, and one N or T, but also with the S misread as F or L. The latter would of course end up in a completely different part of the index and not be so easy to find.
