The Captain
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Definitely a crack. As @Craig B sez, there is a visible step. If you can feel it with a fingernail passed over it, it is significant. IPA and a toothbrush to clean it. Let it dry thoroughly. Spot of gel super glue wicked into it and leave overnight. Some fine sandpaper / emery cloth to dress it which you should slide along the rail rather than across so any minor grooves won't matter.
Hi 'freyman, good to have a 2nd opinion on the crack. Right then, will look up emery paper. I was wondering about superglue & wet n dry sandpaper (without water) ie very fine s'paper myself today. Gel stuff.. good idea that.
Great I'll explore this line of attack, before considering replacing the block (tbh this idea I find a little terrifying!)
Thanks chaps.