A better place for this will be in the configure.php file(s)
It actually is there. I tried simply redefining it at the top of ozpost.php but it still broke, I had to put it in the two lines mentioned to get it to work. Thanks for the extra info RE weights and dimensions, I'll get those too. I realise it needs updating but I had to modify so many core files, it's a bit of a pain.

Happy days, I can now have a glass of red...