On Andrews & Arnold's website I don't see an option to calculate the units-based tariff charges for downloaded data differently for 20CN vs 21CN or TT-LLU users any more. It used to be that if you selected 20CN, it showed you a data tariff charge that gave you half as many GB download per charging "unit" during peak hours (0800 - 1800 Mon-Fri) compared with 21CN.
So have they now made 20CN the same? If so, then good news, it halves the peak daytime per-GB costs for users who are on the traditional units-based tariff. Although the tariff calculator page seems to make no mention of 20CN vs 21CN any more, elsewhere on the site it still suggests there's a cost difference, which I suppose could be just old text that hasn't been updated.
It's surprising though given the recent warnings about the costs of 20CN, see older thread at
http://forum.kitz.co.uk/index.php/topic,17839.msg324300.html