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To be clear, I like to be able to safely touch the ground while sat on the saddle.  I can then easily restart riding again.
I will try sliding forwards at stops - but I do find that position can get a bit unstable on restarting, particularly if I need to make a turn or have luggage on the back.  For me the stand over height is not too important as I usually launch and demount with a leg backwards.

I have now fitted the Sherpa fork. Offset on that fork is 46mm (is this the same as trail?).  I must measure the offset of the Raven fork.

The BB height is down from 295mm to 280mm.   The lower height feels fine.

Visually the Raven now looks…..like a Sherpa with a much shorter steering tube (the tube in which the fork stem goes through).  I only had a short ride today but it felt OK - perhaps more responsive than with the original fork?
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Got it.  Not a top tube problem.

Since almost all bikes now have sloping top tubes and use longer seatposts than years ago, stand over height is almost never an issue any more.  My rando bike has a horizontal top tube, but the rest of my bikes have sloping.  Half a century ago bike shops often sized bikes by wanting just enough stand over height and maybe a tiny bit more for safety, but that was about it.  And a fist full of seatpost (roughly 100mm) between the frame clamp and the seatpost clamp was about right.  But now, often have more than twice that much seatpost exposed.

I think most touring bikes have low bottom brackets compared to some road bikes or cyclocross bikes because on touring bikes you never are leaning over for tight cornering.  And lower center of gravity for stability.  That said, my Nomad Mk II feels like it is very high compared to other touring bikes, I assumed that was by design since the Mk II is intended to also work well off road where pedal clearance on rough terrain can be an issue.  Attached photo, on this trail, many times the trail was so eroded that I could ground a pedal while pedaling.  For that reason, I wished my bottom bracket was a bit higher.

If you extend a line straight through the headset to the ground.  Then measure the distance back from that point to the center of the tire patch, that is trail.

Long trail favors holding a straight line quite well when riding, short trail will make a bike more responsive, but if you take your eyes off the road a short trail can result in you going off course easier.

That is roughly the extent of my knowledge about handling in bicycle design.  If I got that wrong, I am sure Dan will correct me.

If it feels ok to you when riding with this other fork, then I would not worry about it.  Go ahead and use it.

I always thought that the Raven and Sherpa were essentially the same bikes, Raven being Rohloff and Sherpa being derailleur.  So, I am a bit surprised to learn that they use different forks.

I have not ridden my Sherpa for about eight years, but it is a good bike so I have no desire to part with it.  I may use it on another tour again if the right circumstances arise where I would prefer a derailleur bike and would prefer one that is more robust than my titanium touring bike.

It is my understanding that all of the step through frames that Thorn made were the smaller frame sizes.

I am about 6 ft, my Nomad Mk II is a frame size 590M, not step through.  It is not for sale, I am only commenting on sizing, as it sounds like we are about the same height.

The pinch bolts on my bike have hex heads and do not have enough free thread to add locking nuts (I assume that is what jam nuts are?).  Perhaps bolts with grub screw ends would be better?

I will try mickeg’s inner tube binder solution as that obviously works for him.  I must also carry a 15mm spanner!

A couple of additional thoughts from me:
I've assumed it's saddle to ground that we're talking about, Thorn bikes have pretty low TT so it's be seriously the wrong size of bike for that to be an issue.
i don't understand those suggesting thicker shoes?  Wouldn't you have to raise the saddle to maintain the same position, in which case you might as well just lower it.
it is good practice on a loaded bike to come off the saddle when you stop, even so I'd still like to feel safe and stable while in the saddle for times it isn't possible.
Thorn don't publish head angles, but there isn't a lot of variation. 10mm fork length, with the same offset, alters it by about half a degree, measuring offset is easy (Measure, turn 180, measure again, half the difference)you can play with the numbers here:
 http://yojimg.net/bike/web_tools/trailcalc.php
Touring bikes are usually have train in the range of 50 - 70, you have the kit, you may as well try it, but I wouldn't like to stray too far out of that range.

Which is it?

That's a good question, George. I figured if I covered the other end, we'd nail it helpfully either way. 

Best, Dan.