Hello NZkiwi1 and welcome to RootsChat.
rachelralph has not been active here since March 2017, but hopefully she will receive email notification of your post.
I don't know why the name change, however I notice that on FreeBMD there are multiple entries for Richard and Margaret's marriage with surnames:
Richard T DENIGAN and spouse surname WATSON or DODDS
Margaret F B DODDS and spouse surname DENIGAN or DODDS
(Dec quarter 1947 Southampton 6b/1347)
The marriage certificate should provide some answers.
