That look an absolute bargain Lewis, if it was my size I'd be tempted, even though I have no use for it. Just a couple of things:

Cranks - 65mm

Either you have really short legs, or there's 100mm missing there 

Tubus Ti rack

Those racks are currently an astonishing £320.  Unless you've already had an offer, I'd be tempted to take it off an sell it separately, they last forever, and mine still looks like new.  I think you'd get half price, £160, for it, yet it won't add that much to the value, unless someone particularly wanted one.
Good luck with the sale.

Me again.  Thorn Audax Mk 3, size 555S.  Brit Racing Green.  Purchased new from Thorn 2017. All receipts available

I am 5'8" tall, 1740mm.  I find it an excellent fit and comfortable bike.  Current set-up relatively upright, long steerer tube gives plenty of opportunities for change.
Cranks - 65mm, Thorn Chainset.  Deore Cassette.  48/36/26   10 sp 11-36.  Will be sold without pedals, temporary pedals can be fitted
Frame 858 Thorn tubing, Reynolds 853 forks.  DT Swiss R460 wheels, Scwalbe One 28-700 tyres. 
Tubus Ti rack, pump on seat tube pegs.
Currently with bar end shifters, original brifters will be included.
Shimano BR-650 caliper brakes
New spare cassette available.

Pricing is difficult in current market, £500 ono?  Collection from Sheffield only, or meeting elsewhere by arrangement.

Reason for sale - getting older, now prefer Sherpa!  This bike has served me very well, never even a puncture, but needs a new home - hardly ridden it since pandemic.

Photos in preparation

Lewis

I am aware of a few people on the internet that use a Nelson Long Flap or a Camper Long Flap for credit card touring, where they bring no camping gear, they are sleeping indoors and eating in restaurants.

I am aware of one person that has used a Camper Long Flap and an extremely light weight tent and sleeping bag, he was primarily credit card touring but occasionally camped when he was unable to rent a room for the night.

I bought a Camper a few years ago to have for credit card touring, but have not used it.

I used my Nelson Long Flap on one tour, it was supported by my Ortlieb Back Rollers underneath it.  Photos attached.  I mostly used it for food on this tour.  It worked well because I could easily vary the amount of food in it.  To give it more structure to hold its shape, I put a piece of cardboard inside the Carradice bag.  The cardboard worked well enough that I later replaced it with a piece of Coroplast which essentially is a plastic based corrugated cardboard.

But, I have generally chosen to use a dry bag instead of the Carradice bag since that trip, the time to thread the leather straps through the saddle were much more time consuming than other options that now use.  On my touring bikes I use a Brooks Conquest (or on one bike a Brooks Pro).  These saddles lack saddle loops, but the conquest has springs and I can attach the Carradice bag straps to the saddle spring loops.

For several years I have used a Pendle on one of my bikes for day to day use.  I also have a piece of Coroplast in that, last attached photo, the Coroplast is white.  The Coroplast is not attached to the bag, it just sits in it loose.  The bag has a much better shape when not full with this to give it more structure. 

If you try something like this as a stiffener, I would suggest first trying paper based cardboard from a box you plan to discard to see if you are happy with it first.

I can't answer your question, but reach issues can often be solved with a different stem.  Handlebars also have varying reach.

I no longer try to judge bike size based on seat tube length (or effective seat tube length) alone.  I mostly focus on reach, and a secondary issue is making sure I have adequate stand over height.  That said, now in the era of sloping top tubes, stand over height is rarely an issue any more.

Starting a bit over a decade ago when I ordered my Nomad Mk II frame and fork, I based that almost exclusively on effective top tube length and stem length when I was assessing which frame to buy, I already had a couple bikes that fit me well so I could take measurements off of those to compare to what I wanted to buy.

And after I buy or build up a bike, I often do some fine tuning with different stems.