Where to start ?

[Dancess]

Having finally decided on track plan, mine is a terminus, and the layout of fiddle yard there comes the question of which end of the layout should one start building from?

[Newportnobby]

Are the scenic section and fiddle yard on separate boards? Apart from wiring the fiddle yard is a breeze compared to the scenic bit. If they are on the same board I would loosely lay all the track first just to make sure your plan fits and that you don't have to make any compromises.
Assuming you have a scenic break (road overbridge, tunnel, buildings etc) into the fiddle yard, mark that clearly on your board and then go for the scenics, I would say.

[port perran]

I'd pretty much agree with what Nobby says.
I like to lay track first then start on the scenic areas. I divide the scenic up into areas and like to finish (or nearly finish) one area before starting on another.
Generally, I wouldn't want to be leaning over a completed area so probably start at the areas hardest to reach.