Geronimo !! :D
Thanks Nigel.  You may be able to see that it forms part of the bunny barrier and will be open to the inside of the horta.  A couple more courses to go and a sloping top with lids.  I'm also going to render it.  I'd thought about waterproofing the inside faces but hadn't thought of pool paint.  I've got some left over from painting the pool.
I'd thought of feeding a dribbler into the top of each bin from the irrigation system so it they get a bit every time we water the plants.  Can't be any worse than the UK way.  ;)