Outer fabric is...
two layers of pure cotton fabric sandwiched with Butyl synthetic rubber.
(This resists a higher range of temperatures, and will not harden with age...)
Dirt and stains
for removing things like pencil marks you may want to try plasticine putty: called ‘tic-tac’ in Anglophone countries, this is used to stick photographs to walls (and if left too long leaves oil stains on both!) Dab repeatedly at the mark - the putty tends to penetrate the grain of the fabric, literally pulling out the offending matter, which it promptly absorbs - like a dry sponge.
Assuming this doesn’t work, move to plan two ...
Mild soap
a solution of natural soap, applied gently with a damp sponge - work in the direction of the weave of the twill, wetting the whole of the area affected - for instance if the soiling is on the lid of the bag, be sure to wet the whole lid, so that in drying down no differentiation is evident.
Likewise, avoid wetting the leather, so far as possible, but if this is inevitable then be sure to dampen lightly the whole of the piece affected and not simply a small section.
‘Rinse’ by progressively reabsorbing the moisture with the sponge, or if you prefer, with absorbent paper towel.
Leave the bag to dry naturally, out of direct sunlight.
DON’Ts:
don’t use chemical solvents or bleach of any kind -
don’t try the washing machine either - differential shrinkage of the linen lining will occur, but more importantly, the leather will tend to dehydrate dramatically.
Especially don’t use the tumble-dryer! (We know: we tried)
Fading
like all natural cottons, over time the fabric will fade on exposure to sunlight. This is a relatively long-term, gradual process, and together with a darkening of the leather - due to natural oxidation - you’ll note that your bag will be quietly leading a life of its own, displaying the evidence of its rites of passage in acquiring the face that it deserves.

 
Brass buckles and fittings
Brass is one of the ‘noble’ metals, and thus threatened - commercially endangered. We’ve struggled valiantly to resist the tendency towards high-impact ballistic nylon injection-moulded two-ton breaking-strain banal buckles and fittings...and instead bring you items from an older civilisation than our own, cast in sand-moulds in India, polished by hand and wrapped individually in little cellophane packets by folk comfortably sitting cross-legged on the ground...
Brass will tarnish, turning dull brown in full solitude, but equally burnishes brilliant gleaming gold when used frequently, reflecting the scale and range of your activities...
No, we don’t recommend any other kind of polish.

 
 
 
the leather
Traditional, natural, full-grain, vegetable-tanned, drum-dyed and oil-rich.
Hand-made by a small family tannery, ‘les freres Ciulli’, in Graulhet in the department of the Tarn in France - for centuries a centre of the leather trade.
In general, ‘modern’ leather involves sophisticated chemistry and processing for treatment and dyeing - in an attempt to make it resemble ‘synthetics’, ie impervious
to the effects of aging, wear and stress.
By contrast, traditional vegetable-tanned leather remains ‘organic’ in that it acquires a distinct patina of use. Assuming the surface has not been hermetically sealed by a varnish, the leather will darken and display the natural effects of wear and stress, and react visibly to the nutritive treatment given. (The black leather however is a case apart: the black dye tends to fade over time, and definitely requires revivifying occasionally with a dye-polish mixture - produced for shoes and usually supplied with a natty little applicator.)
Nourishment being the key word: a high oil-content in the leather will help in retaining suppleness, resisting dehydration and splitting or cracking , many years on.
You may however choose to ‘neglect’ your bag, with no great risk: the leather will look after itself relatively well - barring desert conditions or extreme humidity.
However, an annual treatment with Dubbin, mink oil, or better still Neat’s foot oil, will work wonders: use a cloth wrapped around your finger - or a small paint brush for the oil. Scrupulously avoid touching the fabric - obviously - and be especially generous in treating the leather tags that couple to the brass collar studs - though we’ve doused them heavily, these need good nourishment to remain supple.

 
Linen lining
Very hard wearing, and with a remarkable ability to resist friction, linen is used in France by La Post Office for the huge bags that hold all the mail. These are dragged along the floor in the sorting room, and heaved into vans and airplanes - they last for years despite concerted abuse...
Linen has a very low loose-fibre content too, and apparently is NASA’s choice in polishing the mirrors on the telescopes in their space programme...
For our purposes, the linen lining in our bags brightens the interior by about three stops - important when you need to find that lens before the bird has flown...
Incidentally, you may come across the most extraordinary things lost in the bottom of your bag, and from time to time it is a good idea to use the vacuum-cleaner, particularly down in the seams where dust has a tendency to gather in social groups...

Humidity is bad policy for camera bags - and in fact most everything else excepting fish, earth worms... and boats (in a sense).
Those funny little sachets of ‘dessicant’... are they still available? Good idea, and equally a good idea to keep your bag out of dark cupboards - especially out of reach of camphor balls!