Mike:
yes I think you are probably right about the second magnet being used for detecting revs also probably to detect direction as well.

Extra_300_S:
I am going to modify mine for the time being because I think it will be some time before MVVS send out the replacements etc. [&o]

what I think we need to do is make a spanner of sorts to hold the drive washer (prop driver) which will need to be a piece of Ali/steel bar about 2 inch wide with a 1/2 inch slot cut in the end to fit around the shaft then say 3 or 4 holes to align up with the existing holes on drive washer. This will enable you to hold it tight and stop it turning whilst you have two 10mm nuts locked together on the shaft (don't forget they are 10mm fine thread), use another spanner to undo the shaft, but as mentioned by someone else here it will be tight and may need a little heat to release the threadlok if there is any. I am also assuming the shaft is on a normal right hand thread. Using heat on the shaft like this means you will have to be extremely careful not to over heat otherwise damage will occur to the bearing seal etc !!

Any better ideas welcome

regards Ian