So it turns out the internet is awesome.
After I posted a python script that can be used to convert a .csv of
Northings and Eastings in British National Grid to a Latitude and
Longitude, several people followed suit and sent through their codes in a
variety of other languages.
I’ve just changed over my website and didn’t want to lose the codes, so
I’ve put them all here in one handy place. Hope you find them useful.