Cheapest flex in the old system was a W class fare, now the cheapest flex plus is R class, hence the price increase. Fares don't seem to have changed on a fare bucket basis.

In terms of getting the full range of fares - it seems to depend on your initial class selection. If you select "Economy" you are shown Economy Saver, Flex, Flex Plus and Business Saver. If you select "Business" you are shown Business Saver, Flex, Flex Plus and First Saver. If you select "First" you are shown First Saver and First Flex Plus. Note if you use Advanced Search you don't get any choice in which fare class you can buy at the moment. You are given the cheapest for your class of service.

Incidentally, First Saver is cheaper than Business Flex Plus on some routes! And the mileage earning on First Saver and First Flex Plus on say, LHR-BKK is 19497 and 19500...I think I know which one I would go for, considering the restrictions are 300 GBP for refund and 200 GBP for changes whereas Flex Plus has no change/refund charges...!