hahah, I was literally looking for the exact same stuff in my Tesco's on Friday night. I eventually gave up! All I can ever find in Tesco is the perishable marinated blocks of Cauldron stuff.
And then last night, I was in the little Chinese/Japanese/Korean supermarket and they had blocks of shelf-stable silken tofu (firm or soft) for 99p each so I just bought a bunch there.