If you tell us exactly what certificates you are seeking, and give us names, dates, and any additional information we can have a try at narrowing down the most likely certificates if you like.
If you have the people on any censuses it would be useful if you could post those details too.
Added: Like Lizdb, it's been a while since I ordered any certificates from the GRO, but a few years ago they did make some changes (and now of course the mother's maiden name has been added to the index of births which helps enormously). If my memory serves me correctly I think you can request a search of a range of years, so +/- x amount of years. Being a cynic I am not sure how thoroughly some searches are conducted though.
You can also go down the local record office route - I have not done this, but a lot of people find that the staff at some ROs give a more personal service. Some have said that if you ring them and request a particular certificate for a particular date and specify parent's names for example, they will tell you if that is correct or not. Note that you do not use the GRO reference numbers for these certificates but you need to locate them via this site I think:
http://www.ukbmd.org.uk/
