Rotten luck but glad you didn't fall or get hurt, Matt; lucky it was noticed at a dismount.
You'll remember I have replaced some fractured saddle rails and re-riveted the cover to the rear cantle plate and nose insert.
Your repair is much easier and more straightforward and can be accomplished in moments. I fit the shackle and then run the tension pin through the shackle's holes, running the pin nut fully home to achieve as much slack as possible, then use the thin prybar much as one would a shoehorn (which would probably work as well, provided it is tempered steel) to slide the end of the tension pin onto the hole in the shackle. Tension to preference but don't overdo it. Older saddles can sometimes run a bit dry and it is easy to create a tension fracture/tear in the leather if one is overenthusiastic.
As for continuing your ride on a known-broken Brooks...
It is of course up to you and your risk valence. The tension pin and shackle do, after all, tension the saddle's leather cover. I'd be concerned it could get swaybacked to an untoward degree if ridden untensioned.
A cable tie or such won't help in this case because you need to push the nose away rather than drawing it nearer. It is not terribly difficult to form and machine substitute parts that would get you by, but I don't see much chance of that on Orkney. Don't know how close you are to Kirkwall, but it seems there are a number of shops and services devoted to repairs there who might be able to help in a pinch. Alternatively, there appears to be a cycle hire shop -- Cycle Orkney, also in Kirkwall -- that has bikes and accessories for sale and possibly does repairs. I'd be inclined to see if they might have a cheap "get you by" saddle so you can save your beloved Brooks for a proper repair at your leisure.
Wishing you well, Dan.
