The Aquatic Warehouse in Hertford had some in the last time I was there. Shame my tank isn't ready for them because it is the only place I have seen them.
There is another lfs in a garden centre near Hatfield on the way to St Albans that must stock them sometimes too because they have a little information label and picture on one of the tanks. Never actually seen any in there yet though. I can't remember the name of the garden centre but I could give you directions if you wanted to take a trip up this way.