Update, I received the substitute cluster this morning, but the speedo glass got damaged in transit!
At this point I figured, I've got a bad speedo with good glass, and a good speedo with bad glass, and I was reasonably confident I could swap the fronts over without damaging anything, work has been slow, I'm up for a challenge.
I masked over the bezel and then clamped it with a hose clamp to stop it from deforming, then went round and round and round with a jeweller's screwdriver.
If you want the ideal tool for doing this, get a 2mm flat head jewellers screwdriver, cut the tip off, then grind one side flat. So you end up with this:
Good speedo liberated from cracked glass:
Aaaand done:
Happy days. I've sent back the cluster with the error which now also has cracked glass. I left the bezel loose to make the next guy's life easier. I think you could make new glass as you can buy instrument glass online in 1mm diameter increments, you'd still have to be supremely patient to drill out the hole for the trip meter reset though, and probably need some quite nice diamond tip bits (by "quite nice" I mean not the kind you buy at Bunnings).
