Witnesses at Weddings
It is important to realise that when a couple were married in church from July 1837 onwards, they signed in the church register. The civil copies - which you get from the local registrar or through the GRO - are copies made by clerks. This means that the signatures on a marriage certificate from the microfilm of the church marriage register can be used for identification - in the same way as signatures are now used on bank cards, etc. For instance, by comparing the signature of a witness at a wedding with the bride at another could help to decide whether they were one and the same person. While many people who could not write simply made a cross as their mark, some people had a distinctive mark - so this is worth checking as well.
When you are looking at a marriage register it is always worth looking at the other entries. Sometimes a church warden would witness a fair percentage of marriages - and a glance at the register could show that your "mysterious" witness had also witnessed other many other marriages so was probably not a relative. In addition witnesses are not indexed on the IGI and many other indexes - and a scan for the surnames you are interested in among the witnesses could suggest new surnames which may be related.
If you can add to the information given above tell me.