Boxer Mechanical Speedo Drive Failure

The Speedo has packed in on my 2001 Boxer Symbol and although it has not as yet been exhaustively diagnosed, it COULD mean the toothed plastic speedo drive-ring within the diff will need to be replaced. Diagnosis so far is that the speedo cable has seized and this will have caused the teeth/cogs on the speedo drive to strip.
I have read that taking the gearbox apart is the only cure, although I have also conversely read that it can be done without a strip-down... because the internally located drive ring is steel and it's the externally accessible plastic cog on a shaft that gets worn down. 

Can anybody confirm the correct repair procedure?
Has anybody had this job done? 
How much did it cost them?
What did it entail?
Thanks