Your grandfather's birth was in fact registered in 1931, and then re-registered in 1945
Births, March 1931
Bristol vol 6a page 138
Blake, Trevor J. - mother Blake
Smith, Trevor J. - mother Blake

The birth is indexed under both names, Smith and Blake. So presumably both Ronald + Doris were present at the birth registration, Ronald declared himself Trevor's father, and they both signed.
There is a handwritten note at the bottom of the page for the Trevor J Smith index entry, see J'45

Re-registration
June 1945, Bristol 6a 148
Smith, Trevor J. - mother Blake

John