Wow, thats a huge wicking bed englyn! You'll be able to grow heaps of stuff in it.
I'm still mystified by the vagaries of gardening. After getting a bumper crop of lettuce, I 've using exactly the same technique for about 2 months now, and nothing is germinating. I am using a different pack of seeds, so although not out of date, maybe they don't like what I'm doing. Just resowed some of the previous variety in desperation. I sowed heaps of carrots also, and only have half a dozen seedlings. Basil has germinated - i thought it wouldn't be hot enough, I usually leave it til a bit later, so there you go. I have managed to germinate 2 cucumber plants from some out of date seeds....early days yet - still 2 leaves - before I'm sure they'll survive.