For starters, here's a scale plan and elevation of the loft (Visio is
really starting to do my head in for this sort of stuff - can anyone
recommend a more 'appropriate' package for plans, elevations and (if
pos) 3D views?):-

I often ask myself that - there must be a package that is more
oriented toward building plans and layouts without going to full
blown CAD. When you find it let me know!

The package I have used for years is a pencil, rubber, T square,
drawing board, paper etc. I find its much quicker than any computer
prog and a fraction of the price. The drawings are somehow much more
readable too.
Recently bought a posh board with parallel motion 2nd hand - only £40.
so its even easier.

cheers

Jacob