Well, new criteria showed up over night. I kind of specified it in the text but not in the sample data (a cardinal sin).
With a slightly clearer brain today I went googling and found this:https://www.mrexcel.com/forum/excel-...haracters.html
I also decided I wanted to end up with 5 characters regardless so I used Sandy's BASE trick.
Now with everything solved I got an existential crisis instead. Is this really what I want?
I don't think so. Even if this formula allows me to sort my PO no's correctly in a table with a helper column (a rather unattractive option to start with), the pdf's generated will still not sort correctly in the folder.
The only sensible thing to do is to use three digits for the running numbers to start with 001, 002, 003 etc.
I am reasonably good with formulas myself but string manipulation has always been my Achilles heel. This session has helped a lot thanks to your inputs. 
For future readers, this is what it ended up with before I decided to abort the whole idea:
Formula:
=BASE(TRIM(LEFT(SUBSTITUTE(MID(A2,FIND("|",SUBSTITUTE(A2,"-","|",3))+1,LEN(A2)),"_",REPT(" ",LEN(A2))),LEN(A2))),10,5)
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