Finally got the chance to post a follow up to sorting the speed sensor out (and passing MOT win!):
So here's my sensor after the retaining bolt sheared off and I discovered the sensor still firmly held in place by corrosion:
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I then drove an old screwdriver down between the metal plate on the sensor and the mount. Not the best move in hindsight as it damaged the mount slightly and the right hand side (as viewed in the photo) of the sensor sheared off leaving the cylindrical body still inside the mount and projecting in towards the disk through the mudguard/spray shield thing. I ended up getting a hacksaw and chopping up the sensor like sausage and extracting it in bits back towards the mount. This left me with a sheared off bolt in the mount and a cross section of the sensor inside the mount (sheared off on the right and hacksawed flush on the left). Got a punch and drove the final bit of sensor out of the mount and a flexible drill extension to drill out the bolt:
Flexible Drill Shaft 915mm - Toolstation
The bolt's thread and the mount were so rusted together I ended up having to re-tap the m6 thread with more cheapo toolstation kit:
Tap & Die Set 10 Piece - Toolstation
Here's the mount cleaned out:
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I got a new bolt from Audi (19p) part number P AN 014 740 11. Audi wanted £105 for the sensor but eurocarparts got me a replacement bosch one for £63. The eurocarparts number is 417 44 0430 and on the packaging it seems to show the bosch part number as 0 265 006 681.
I filed down the inside of the mount so I could slide the new sensor in and put some copper grease around it to hopefully stop it seizing in place. Here it is (you can see the slight gap at the bottom where my screwdriver antics chipped a bit off the mount):
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One thing that's worth pointing out to someone like me working on a car raised on axle stands is the gland where the sensor cable enters the body is a sod to push back in place lying scrunched up under the car. I loosened the plastic cover by taking off the bolts on the leading edge of the wheel arch to improve access a bit enough to take the old sensor out but I think you'd need to take the cover off completely to push the new one into place (I got the garage to pop it home when it went for it's MOT a few days later). Also I pulled through a shoe-lace when removing the old sensor so I could use that to pull the new one up through the body into the interior. Here's the offending gland refusing to be pushed into place:
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If I'm feeling very diligent I might remove it again and chuck some primer on the mount to stop it rusting so quick.
So last thing that I'm hoping some pro on here can answer is what actually went wrong with my sensor. When I extracted it I noticed that there was some grey gunge on the end of the sensor on the magnetic core that's exposed at the end. I'm wondering if this was some lithium grease that was conductive enough to short the core to the disk and ground the sensor reading? I've never greased up anything on the hub/wheelnuts but we did get the rear disks changed last year so I'm wondering if in the heat of france somehow the grease flowed out from wherever it was and got on the end of the sensor?
Also if this is why the sensor started playing up why is the end of the sensor exposed at all? If there's no good safety reason I might try to cover the end so it can't happen again!
I think I've read on here about 'cleaning' up the sensors but I can't seem to find the post now so I'm wondering if this is what they referred to (making sure the bit of bare core isn't grubby) and that also ties in with what doctle odd said about checking the ring above before removing the sensor. I assumed you'd need to take the whole brake off to get a look at the disk and ring surface so as I needed to keep the car running and haven't played around with rear disk brakes/hand brakes I was a bit intimidated so thought I'd just see what I could do just taking the wheel off. As there's no other obvious signs of damage on the sensor if it was just this shorting of the core to the ring by the grey gunge then perhaps the whole thing could of been fixed by removing the disk and wiping the end of the sensor?!
Any thoughts welcome on this and I hope my struggles save someone else some time in the future
Ian