British Embassy states here the following:
The birth may be registered at the Civil Register Office nearest to the place of birth or residence, by one of the child's parents. This should be done in the first 8 days following the child's birth. However, this period can be extended to 30 calendar days if there is a justified reason for the delay. If you miss this time limit with the Spanish authorities, you may have problems getting the birth registered. You also may have to start an administration procedure with the Spanish authorities which will be long and expensive. This can affect your child's access to state health, nursery or school.
Also check out this thread (the last couple of pages should explain the situation) - you will see some of the problems Vicci is facing for, unfortunately, being a few days late registering her child.
Maybe someone can tell you where the nearest Civil Register Office is to where you will be living (I think you may be in the Manilva area?).