We might have a slight angle on the shroud at the spreader as you suggest although it is quite hard to see. With the rigger we are going to loosen everything off and "start again" on the whole rig making sure everything is in the right plane, angle and tautness. He also does not think the mast shape is not quite right so we will address all of this to make sure we have everything as it should be. Having inspected the area very closely he thinks we caught this very early and the cracking really was just in the paint. With the paint removed from the area there is absolutely no evidence of cracking in the spreader or damage to the weld and he thinks it isn't necessary to penetration dye test the area. There was obviously some movement to crack the paint (very very minor on the port side) but I (and he!) think we caught it early enough to avoid any real issues.
I am going to completely retune the rig, re-paint the area and watch it VERY closely.
Thank you Alain for your extremely helpful information and input and indeed everyone who contributed on the subject. Finding it was so unexpected I hope I may have got a bit over excited when I put "Spreader Cracks" as the subject at the start of the thread!