As hoped, I've at last been able to do some work on the new loop. The alignment is more or less (barring the usual tweaks) in place at both ends of the loop. Here is the loop junction on F5, with track added for illustration.
And here's the new spur on F1 looking towards the back of this section (the section is currently resting on others along the back). Not a good photo; sorry.
On F5 I now need to realign the outer trackbed (F5o) as there's no longer enough clearance where the new loop spur crosses. This means starting and, of course, finishing the outer gradient further round the loop.
The other casualty of the F5 realignment is my neat little branch line feeder than snakes to and fro among the trackbed supports. The newly moved gradients now don't allow clearance for this any more, so I will reluctantly have to move it. The easiest solution will be to take a spur off the F5o mainline feeder, just after it emerges from under the new loop spur. This might also soften the gradient on the branch line, so it's no bad thing (although a less interesting alignment).