Looking at the parts manual I now know what has come loose behind the pulley, part 2447766 described as a "mud excluder" and held on by 8 screws, all of which have obviosly come out.
It looks an awkward bugger to replace cos I can't see how to get at it without removing the radiator to remove the pulley unless I had extremely long arms and amazing agility.
I assume it is safe to run for the time being with this thing off, so long as I am not going through any mud,
any suggestions?