I wonder if it's the same one as on the SW website? I frikkin *love* that carrot cake (so does my hubby, which can be problematic!) - I can double up the recipe and still eat half the (massive) cake and if you count the scan bran as a Hex-B it's still only 6 syns. Amazing. I made it for my SW group at the Christmas party, and it went down a treat!
Try looking in any health food shops. There's even one in the local market here that has them, plus Holland and Barret, Health and Herbs - they all have them here.
We haven't got a waitrose but I've heard that they sell them too.