Well I was prompted by this thread to take my b31' s chronometric apart, as it had a similar issue, with the 10, 100 and 1000 numbers jumping up every 10 miles.
I found the first of the five number reels had a worn "ratchet" on one side which caused it not to lock in position as each mile ticked over, this caused it to move the adjacent 10 mile reel and so on.
As a temporary fix I moved the duff reel to the 10,000 mile position where it will cause less harm.
I don't know where I will find a replacement reel! I guess I might end up having to take it to a specialist speedo fixer, or keep having to reset it to back to zero again every 10,000 miles
It was VERY fiddly getting the 5 reels back in position,
to those speedo specialists!