Right! Time for the final version (except for the later versions when I change it all again.)
Besides, I feel a bit like I'm coming down with something so I might not feel like fiddling with it much more in the time remaining.
Thanks to everyone for their advice and ideas. I think the finished product is all the better for that.

As I think I mentioned before I took the challenge as the start of a campaign and an important aspect, I feel, is that that is all it does. The rest of the campaign is up to our prospective players and GM to sort out.

To reprise the information dotted around in this thread...

The Tin Man's Map

This is a deliberately "open" design for a Campaign start scenario. The GM is left with the majority of choices as to how to progress it. Some ideas are presented here.

The basic salient points are
a) the player characters stumble upon the scene in the battlemap.
b) they find the "map"

Battlemap
There has obviously been a fight here and the wagoneers lost. The scene has already been looted of horses and/or mules and all major expensive and/or small goods.
The GM needs to decide where to place any bodies and active combattants that might be around now.
It could be that the original brigands have returned to load up more booty on the horses. The carts themselves are largely useless to them as they won't pass between the trees around here. It may just be scavenging wolves or other itinerant looters (like the players?)

The fundamentals are that the Tin Man is some sort of trader from afar who has found a good supply of Tin somewhere. Tin is a rare commodity where he hails from and has traded 4 crates of Tin Ingots for gold (which is more abundant for him). He obviously intended to "make it big" taking this home. Clues to this could be identified by his tin "jewelry" and the use of gold in some everyday utensils that may remain. Perhaps the brigands did not believe that the mugs had real gold bands on them?

The Map
Is cheaply hand scribed and has seen some wear and more lately, some blood. It would most probably be found hidden in the Tin Man's robes. The language is obviously "foreign", ideographic in written form, and translators are probably scarce (if any) or perhaps a survivor remains. Some smiling little asiatic old woman stereotype??? There could still be a language problem.
Even then, the ideograms could be in "code" known only to the Tin Man.
The map "should" be read from bottom left to right then up "one" and left to right again, ending in the top right. The players should be able to work that out?
The map itself has some diagrams depicting possible perils (skull and crossbones, bows, axes, snakes etc) some of the more interesting elements are enlarged in adjacent copies ("stonehenge", rope bridge...). Crops, livestock and game are pictorially shown, perhaps to assist in smalltrading along the way?
Navigational aides are sometimes shown by the sun (either at sunrise or noon) or moon and an arrow. It might be that holding the map flat before you and aligning the sun/moon along the arrow might mean you are facing the correct direction? Sometimes the horizon is drawn at rightangle across the path to show a reference point to head for.
Finally - other things on the map warrant investigation? Maybe the little old dear can say a word that everyone recognises - and point... "Ygsdrason!" Nodding furiously! Lots of flapping arms...

The Tin Man himself?
Who is he?
Well he could just be a lone trader looking for "the big one".
There is a black symbol repeated twice on the side of his living wagon and on his banner on the goods wagon - even once on the map!(?). Is that just his trade symbol, or is it the symbol of something more powerful and, heaven's forbid, more sinister? Perhaps it's the symbol of some sort of Trader "tong" or the passport emblem for some "protection" organisation.
If the players decide to complete the Tin Man's trade (and it's likely to be very lucrative) by completing the journey with his carts, are they going to have to talk very fast to explain themselves? Perhaps the ingots all have this mark too?

Take it where you will...

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Technical bumf.
I apologise if it's in anyway grainy - it's a 25 factor compressed jpeg. It should work out as A3 when viewed at 300dpi.
S'all folks!