My securing bolt (like everything else on the car it seems) was corroded to hell, and there was no way that it was going to come out. The lack of available space makes a difficult job 10 times harder. So I've bodged it......... its not a solution that I recommend, but it might get you out of trouble if you have a similar problem.
The body of the sensor is made of plastic - drill through the sensor body near the retaining bolt, and knock the sensor out from the front (you'll have to remove the brake disc). This should leave the retaining bolt complete with its metal sleeve still in place.
You'll now have to remove the metal sleeve from your replacement sensor - to do this I hacksawed through one side of the plastic (at the 12 O'clock position). I then prised the cut slightly open with a screwdriver and pushed the metal sleeve out of the new sensor. Place the abs sensor in the hole on the backplate whilst making sure that you've lined the securing hole up with your rusted in bolt - then push the sensor home through the backplate and over your rusted in bolt and sleeve.
Its a very tight fit, so it shouldn't work its way loose, but if you wanted to make it more secure you could always secure it with a suitable product - but remember you may need to remove it at some stage in the future, so don't use anything too permanent.
The replacement sensor was picked up from ebay for £13 delivered, its a pattern part, but looks identical to the original, and I no longer have any abs warning light problems
