Thanks for the kind advice and warm welcome.
@Nucejoe I've tried to give the watch a shake and the second hand remains in the same place (just before the 45 second mark).
It's had a full wind and it certainly feels like it's not holding any power. We'll have to see what it looks like when I can get the back off it.
I'll let you know what movement is inside so I can take a look at the datasheet and get cracking!
Bring on the postman!
The measurements of the original Orient/Seiko mainspring are as follows:
0.95 x .115 x 370 x 10.55 Automatic (Height x Thickness x Length x Barrel Diameter)
The closest replacement I've been able to find and the one I've been using is a Generale Ressorts spring:
0.95 x .12 x 400 x 10.5 Automatic
The only lubrication I've done is the Kluber P125 around the barrel wall, and I do not lubricate the spring as it comes pre-lubricated.
As far as I can remember the bridle of the GR spring and the original spring look the same or very similar. Unfortunately, I cannot verify this as I no longer have the original spring (per usual I mangled it in my K&D winder).
And yes, just to make sure, I have no reason to believe the arbor hook slips out of the eye of the inner coil. If it did, the spring would unwind in an instant and it doesn't. When I wind the ratchet wheel screw using a screw driver (this watch isn’t hand wind able) it feels very smooth and the only plausible explanation, as I see it, is that the spring is constantly sliding as I wind. No mater how many revolutions I make on the ratchet wheel, unwinding always results in 4 revolutions of the ratchet wheel before the power reserve is depleted.