You should be able to check the page image for yourself on FreeBMD
http://images.freebmd.org.uk/cgi/choose.plIf they were registered as twins then a time of birth would be given to indicate which is the older and younger twin. You do get times of birth occasionally for non-twins, but you should always get a time on the birth certificate for a twin (though I'm not sure that would be the case if one twin was still born - in 1852 nothing would be registered to indicate a still birth - neither a death nor a birth registration. Still births do not appear in the modern index but they are now registered none the less).
There are various reasons for what might seem like a non-registration of a birth. Were the parents married at the time, in which case the birth might be registered in the mother's surname.
Had theparents made up their mind about the child's name. Most births registered as male and female are children who died fairly shortly after their births and never got given a name, at least in the birth registration indexes, but this is not the case for all births. Again as late as 1907 I know of a birth registered just as male. He went onto live well into his 80s and was called Charles eventually, but just not in time for his birth registration.
The child's name could have been mispelt either the first name or surname or even both in the index.
There was a change of name, which can be something simple like Helen to Ellen or from Mary Ann to the pet name Polly, or the person or parents just threw out the first name and only used the second name for whatever reason.
The name could have been mispelt in its male or female form Francis/Frances Jesse/Jessie....
The baptismal name is not quite the same as the registration name.
The birth was registered and if you checked with the local registrar the certificate could be produced, but it never made it into the GRO indexes. I have a birth certificate produced by the Newcastle registrar for 1842 that is definitely not in the GRO indexes no matter how creative I have been in thinking how it could have been indexed. The name is clearly written on the certificate.
The clerk completely messed up the entry in the GRO index - either the name and or district. Again I have a birth in the 1850s where the index gives the name as Joseph George. No birth certificate could be produced for a child with the surname I was interested in with those names at that reference. I persisted with the GRO at Southport because I thought the names should be James Thomas, despite the entry reading Joseph George and in fact below the certificate for the child called Joseph George (and I never did find out what his surname was), was found the child I was interested in presumably with a slightly different page number which was not indexed, the clerk having muddled the two children and given only one entry in the index with the first names of one child and the surname of the other. Since there were no checking mechanisms in place (and none when temps were used to retype some of the older volumes which were in bad condition and subsequently thrown out) the GRO index is prone to errors.
There is quite a list of possible reasons for why a birth can't be found and other people may have other examples (such as aliases).
I presume you have checked beyond FreeBMD since that is an incomplete index (their coverage charts show they have very poor coverage for 1851 but good coverage for 1852)
http://freebmd.rootsweb.com/progressB.shtmlbut have you searched on the subsequent census for your person and also for anyone else in the same district who might have the same name (might be tricky if the name is too common). That might save you some money ordering a certificate which turns out to be someone else.
Regards
Valda