Although the PBC may no longer exist as a handy book, I notice that a similar function is now available from OFCOM
on t'internet, though it doesn't give the level of detail provided by the PBC.
By the way, before my discovery of the PBC, I too spent many frustrating years trying to work backwards from an STD code to the actual exchange. This usually consisted of doing manually what the above-mentioned FORTRAN program did - but somewhat more slowly.