Thanks everyone for your input.

I'm ok with the likely reason being a cutting issue from the factory. And I figure that this is the first guitar where I learn the ropes and just do my best. Everything else is a bonus.

I'm glad that this is on the back of the guitar and I hope that the front doesn't have this problem. But if it does then I'll deal with it then.

I've given it a really good go with some 240 and the line is starting to disappear. Huzzah!!

I think I know the answer to this next question but I'll push on anyway.

The area that I've now been working on is showing a lot more ash (ie is a lot lighter than the remainder of the back) - I assume that I now have to re-sand the rest of the back just as vigorously to match it all up before I throw down another application of Timbermate?

I think I might need to head down to Bunnings and buy a few more more sheets of various sandpaper grits.

Thanks again, everyone. Very grateful.

Cheers.