I may be missing something here ... but how is it you know the date of her baptism if you have not found a baptism record?
Is this her marriage -- 1742 Jacobstow Ann Burdon to Richard Rundle?
http://www.cornwall-opc-database.org/search-database/more-info/?t=marriages&id=642177Hm! for some reason my first search of baptisms didn't turn it up, but a person search did:
http://www.cornwall-opc-database.org/search-database/more-info/?t=baptisms&id=2911360http://www.cornwall-opc-database.org/search-database/more-info/?t=baptisms&id=27985463 Apr 1718, Jacobstow, father William.
At that early date, it was very rare for mother's name to be recorded in baptisms records.
Finding a suitable marriage would be your only likely chance of identifying the parents, but even then you would essentially be leaping over a broken link in the chain and having to assume that they are the right parents.
Here's a possibility, though.
There are other births to William Burdon in Jacobstow; the next one after Ann shows no mother's name:
1720 Arthur
However, the ones following show mother Grace:
Henery 1723
Thomas 1726
Mary 1730
Richard 1733
So it seems a reasonable possibility, at least to investigate further, that your Ann's parents were William and Grace.
Likely, Grace died in 1741 and William in 1747 -- Grace is recorded as Burden:
http://www.cornwall-opc-database.org/search-database/more-info/?t=burials&id=2321622and William as Burden in the BTs:
http://www.cornwall-opc-database.org/search-database/more-info/?t=burials&id=2321622and as Burdon in the parish records (? I assume):
http://www.cornwall-opc-database.org/search-database/more-info/?t=burials&id=1261761but no ages are given.