My idea of a Gravel bike is something pretty close to a drop bar road bike with larger tyre clearance, which then usually leads to disc brakes whether you want them or not.  If I were in the market for such a bike, I don't think anything in the current Thorn range, or a Rohloff, would be on my wishlist. I'd probably choose derailleurs, shifters only lasting 8 months isn't the norm, decades is more common. If you wanted more robust, I'd look at the CUES range of components, designed for longevity, partly with E-bikes in mind, the concept seems to be more function than fashion, though I have no experience.
If you still want a Rohloff, you could just stick a tensioner on your current bike, not as good as a purpose made frame, but not much of a disadvantage either. If you want a more Rohloff friendly frame, here's some you might look at - Surly Straggler* (Or  CrossCheck for rim brakes), SOMA Wolvarine, Salsa Fargo or Marrakesh, Singular Peregrine, I'm sure there's others, but those are models I've considered in the past.  You could add the Thorn Mercury to the list, though you'd need 650B wheels to get the tyre width, and I'm not sure it would be the best choice in that role.  One question is what are you hoping to do on it that you can't do on the MTB? 
* It might need to be a previous version of the Straggler, the new one might have thru axle dropouts.

Just came across a Surly Ogre frame, it even has the Rohloff axle plate mounting slot

I had an Ogre, it isn't in any shape or form what I would consider a Gravel bike, fine for what it is, which IMO is a MTB tourer.  It also has a long top tube, you'd need to be careful with sizing if you wanted to use it with drops.

 

For me, a true "gravel" bike should have clearance for a minimum 45C tires (preferably more).  Definitely don't want carbon!  (As mentioned, even a Ti frame doesn't last me a "lifetime!")  So, steel for me.  Just came across a Surly Ogre frame, it even has the Rohloff axle plate mounting slot.  That looks like something that would work.  Not sure I want to go through the hassle and expense of building up a bike that I'm not using too often.  Probably keep searching for something used between now and spring time.

Thanks for your help mickeg and RonS!

From a Google search I found someone on this forum who said the mk3 has a bigger tyre clearance but I just wanted to confiirm that.

I will consult the archive of Thorn brochures in the Thorn General section!

The first thread in the “Thorn General” section of the forum has a historical archive of Thorn brochures. Perhaps the info you need will be there if no forum members are able to assist.
Good luck and happy riding.

What is a gravel bike?
...

Andy, I assume you have heard the term gravel bikes before and also pretty much know what it means.

I mostly agree with Dan, the manufacturers needed another marketing idea for a pretty stagnant market. 

My thoughts on "what is a gravel bike", below.

In USA there has been a lot of growth in gravel bike trails over the past decade or two, I suspect more people wanted a bike that had tires wider than the common 25mm tires on road bikes a decade ago for such trails.  And presumably some people were tired of rough riding skinny tire road bikes on rough pavement.

From what I have seen, a "gravel" bike has to have a wide range really big and expensive cassette on a 1X drive train. 

• A lot of people did not like being confused by two shifters, wanted a 1X shifter to make life easier.  (Similar to a Rohloff.)
• Retailers found it was much faster to assemble a bike from the box if the front derailleur did not need any adjustments, because it did not exist.
• They could charge as much for a 1X bike as a 2X or 3X bike, that is fewer components, thus a bit more profitable.
• When the owner needs to replace a giant cassette like that, it is pretty profitable for the shop. 
• The giant wide range cassette was a newer concept, thus the latest fad.  If you had that, you were automatically cool.

A friend of mine had to buy a new gravel bike about five or six years ago.  His carbon bike had developed a fatal crack in the frame, and the gravel bike being a new fad showed he was cool to own the latest thing.  Within a year, two of my other friends saw that bike and they had to be even cooler, one bought the same model bike but with a carbon frame.  And the other bought the same model but with both carbon frame and carbon wheels.  I found it interesting that after a few years two of these three started riding their older bikes more often.

My light touring bike is not a Thorn, it has a titanium frame.  I built it up in 2017.  Has a 3X8 drive train, crankset has square taper crank (decades old tech), a pair of shifters that I first put into service in 2004, and a rear derailleur from the 1990s (but fitted with newer ball bearing jockey wheels).  Has disc rear and rim brake on front because I had an older rim brake fork in storage that had the correct crown race to axle length and rake (or offset), thus I did not need to buy a new fork for the frame, saved roughly $400 (USD).  Fitted with 37mm 700c tires.  But other than the drive train the only other difference between my light touring bike and a gravel bike is that I have fitted fenders to it, and since it was built for touring it has longer chainstays.  (Most gravel bikes have chainstays almost as short as a road bike.)

When I built up my light touring bike, why did I choose a 3X8 drive train?  It is robust, reliable, easy to replace parts, easy to adjust and repair.  Last time I bought a new cassette and chain, paid about $35 or $40 (USD).  It has more gears than a 1X and a wider range.  It is a half step plus granny system, which is an acquired taste, most people would not choose that but I like it.  When I avoid the two most cross-chained gears for each chainring, that gives me 18 usable gears, 558 percent range.  Compare that to a 10 or 11 or 12 speed drivetrain that at most is 520 percent on the 1X.  (Campy has a 13 speed, but it is rare.)

First photo, my light touring bike.  Strip off the fenders and luggage, and what you have is a gravel bike with longer (roughly 20mm longer) chainstays.  (It was early spring, the lake in the background had not yet thawed, was ice and snow covered.)

Second photo, my light touring bike being used for what it was designed for.

I could have put a Rohloff on that bike instead of a 3X8 drivetrain.  I have nothing against the Rohloff on my decade old heavy touring bike (Nomad Mk II), but there are both advantages and disadvantages to a Rohloff compared to a 3X8 drivetrain.  The trips that I anticipate using my light touring bike, I felt I would prefer the 3X8 system over the Rohloff 1X14 system.  I put new fenders (mudguards) on my Nomad this year, but they are a bit dusty in the third photo.