Thanks guys @davebYYPCU @TheCADWhisperer
Seems that is the genesis for any design created from multiple components. Wisdom suggests it becomes my 1st action on any project.
I'm now on video 4 of this series, and have walked into another wall. With the anomaly surrounding the gear dimensions on the left side of sketch 1, I have tried to follow the steps as instructed, but ended up with a very different outcome. The 20deg angle of the 2 lines mirrored around the first construction line will not adjust. If I try to delete dimension and re-dimension, inputting 20deg as the required angle, it will not adjust from the 6.2deg it is currently showing no matter what I try. The sketch is fully constrained now - I had issues with keeping it constrained after the amendment to the circle cut-out on the far right of this sketch in video 3, but that seems to have sorted itself out when I began modifying this sketch. Is there a constraint in there somewhere that I should look at deleting?
Also, the tangent lines on the far left of the sketch I've drawn intersecting the construction line to the left from the centre origin look off. I really struggled to place these on video 3, but assumed (perhaps wrongly) that once all of the radii, angles, symmetry constraints etc were defined, that this would replicate the original in the video. I'm pretty sceptical about my ability to track and trace the cause of this without screwing up the sketch irreparably, and having to start over.
As frustrating as this can be sometimes, the reward one gets from seeing the model come together is a lot of fun, and extremely satisfying. Wish I had given CAD a look years ago, but I guess it's never too late to learn something new!
Thanks