Thanks for the feedback, Gavin and others.

I have not shot in any unusually dusty environments at any time. Pro HiTech in Melbourne told me only 4 EX1s have ever had their prism blocks replaced by them. They did estimate at around $2500. Definitech told me they did not have any record of this problem! However they've always done good work for me on previous cameras.

I will be checking my insurance..

Cheers

Patrick