Mohamed,
Thanks for your reply.
I should have stated that this is a live, working store with shipping methods that have been working perfectly for different zones and different product weights (all in kg), so yes, all of the items in that documentation have been covered with the exception of point 9 (Use same weight .... as the first unit in the configured. This was set as default to lb,kg,g,mg,oz,ozt - I set it to kg,lb,g,mg,oz,ozt and re-tested and got the same "weight too big message".
This week we have been informed that certain products containing alcohol cannot be shipped outside of the country. This is why I enabled the Price per Product plug-in and applied the Block option for these products. It is this plugin, and it's logic which is causing the "The cart items combined weight is too big" message.
As I stated before, it's sister website (offering a smaller number of products) has the PpP plugin enabled and applied and it works perfectly...