Hardiness measurements
 

How is the hardiness of a plant worked out and is there an authoritative
central source from which the various books take their facts?

I'm asking because I've just been reading in an old Gardener's world about
Vietnamese Coriander which is apparently only safe down to 7C. I left mine
out last year, and the frost killed off all the top growth into a pulpy
mess. I planted the remaining roots in a disused area of the garden just to
see what would happen. That area got waterlogged and frozen during the
winter, but this spring it all grew back again, proving it to be fully
root-hardy.

I'm concerned I may be missing out on lots of plants which are described as
tender but could actually survive our winters.

PS. I live near Northwich in Cheshire.