I have created this draft sewing pattern from individual paths that I would now like to combine into a single path. I'm having problems with either joining individuals nodes (join node simply does not work) or selecting the entire diagram and attempting to ctrl+k/combine all paths at once.

Joining nodes works only with the end nodes. But, it is also not possible to join end node with a node along another path.
On the other hand, joining node may reduce the number of nodes. If you select several neighbouring nodes and then hover with the node tool over the desired one - only that one would remain intact. Without hovering, Inkscape would make the approximate curve with only one node placed along that new curve.

In your case, side seam is not a path, but a part of the whole right side (unlike the centre line) and that might cause you troubles with combining.

Thx Maestral that gave me a much better understanding of the node/path joining tools.
Whilst I have your attention, How might you approach a diagram like this where accuracy is very important. The instructions I'm following read something like create point "0",from point "0" measure down 2cm mark as point "1" and so on until all the points have been created. In between creating certain points it asks for them to be joined, however I feel that it may be better/easier to join them all at the end, perhaps by creating nodes in the centre cross section of each point and snapping a path through all points with the bezier tool.

I can`t say more than to agree with your later conclusion - make all needed paths and then combine them together. At least, not prior to seeing the mentioned instructions.

Since you have an idea how joining nodes work, all that you need is to bare in mind your desired final result/drawing. If you make it for your self - all that you can use would work just fine, but if you`re passing these drawings to someone else - it`s better to check what would that final "user" expect/prefer to see.

thx again Maestral, I won't bore you with the whole set of instructions. With your help I've successfully managed to combine the paths by selecting and combining them individually.

I have noticed that that the width of the pattern is slighty off, the distance between "side seam" and "centre line" should be 27.65cm instead of 27.63cm, what would be the best method to widen this object by 0.02cm?

This could be easily related to the width of the stroke. The most simple solution would be representing the shape with the color and without the stroke.

Inkscpae has a powerful zoom in and you may come close up to these needed scales but then again, even if you move the lines to match the desired measurements - the drawing would still not be 100% accurate since Inkscape calculates the width of the stroke with the half outside and the other half inside the shape it self. Therefore, the 100% accurate drawing will be the one with just the color.

Again, the whole thing matters and depends on the final recipient/s.

p.s.
If your drawing shall be printed and used as a template for tailoring business, I`d leave it as is, since scissors would cut trough the line and the measure would remain proper and accurate.

p.p.s.
0,02 cm is the thinnest yet still visible printable line

The patterns will be printed and cut out for my own use. As you say +- 0.02cm is plenty accurate for a clothing pattern when taking cutting into consideration.
Thx again