Right, final design set in concrete now, All criticism is always welcome though!
note that the rads I will be using are going to be 480mm radiators, and will get the thickest possible
This will also be a single loops, not dual. Tidier loop as a single, and not enough performance to worry about with the difference between the two.
Thank you to all who have helped me and given constructive feedback!
Timber is Native NZ Rimu, and will commence building this week!