I don't know where the 6 comes from in 'nCCV(V)6'? Each float is 8 bytes, not 6 bytes.

There are 6 values to extract: minx, maxx, miny, maxy, minz, maxx

so, my $minx = unpack ("d8", substr($geo,8,8)); may work??
I am not sure if f8 would work also?

Both d8 and f8 give fractions that seem nonsensical in this situation (because of the SRSID we are using). f8 gives a negative fraction which is doubly nonsensical.

However, it could be that the decoded data is right and I am not understanding it correctly. My plan is to ignore the envelope for the moment and press on to the geometry data. That way I can feed the USRN into a USRN finder (the link is for the first entry in the SQLite DB) and sanity check the values against a known entity instead of trying to work out the envelope first.