Eventually had to uninstall hotfix to get it back up again.

Found some details on

http://blogs.msdn.com/psssql/archive/2008/09/23/reporting-services-windows-service-will-not-start-after-applying-ms08-052.aspx

but didn't fancy a registry hack.

Moral of story: wait for the service packs

Bit difficult to explain but there is a screenshot in http://books.google.co.uk/books?id=b0dXlx5aww8C&pg=PA113&lpg=PA113&dq=reporting+services+matrix+report+show+top+n&source=web&ots=U0MYGHDUWS&sig=AvFsWubHTQU-6PmHxqjrV6rF44A&hl=en

Basically you are filtering the row group so it only shows the bottom or top n percent of values.

To do this from the front end right-click the group column in your matrix report > Edit Group > Filter tab.

Expression: the value in your total column e.g. =Sum(Fields!QTY.Value)

Operator: Bottom...