Hi Mark & Mcbee,
I've ticked the log trigger expression so hopefully will see next time what is causing any problem, as for the reread? where will I find that, I looked through the settings and went into the trigger editor, but couldn't see it, but that would certainly make sense! Most of the time whenever I add a selection I use the reload trigger function on the trigger itself, but not always, which would certainly account for the randomness of the error!