Before I do anything else I think I'll take the botom pulley off and check if there's any damage to the crankshaft keyway. If not I should be able to get away with a new pulley and key.
It's possible that a water crossing did loosen it off as I went through the Fortesque River in the Millstream NP which was about 6-700mm deep about a month ago.
I'll just have to keep driving the cooks .............Tojo!!!!
Thanks for the replies.
Alan.