A visit to the local bike shop with the 'split' rim has confirmed it is nothing to worry about. Trevor at the LBS told me he often gets new rims with bigger pieces missing at the join. It doesn't cause any weakness and is nothing to worry about he says. Nice to get some honest advice. I hope he is right.
I have to wonder why I never noticed it from new though and if it happened at some point over the last 12 months might it not just get worse? Anyway, I have decided to keep using it and monitor closely for the time being. If it gets bigger I will replace it. I just hope it doesn't fail on my weeks holiday in Wales!