Well, i didn't work out how to stop the full table scan when performing updates across linked-servers, but there is a better way of achieving the required result (as always!).

rather than have an update statement on the remote server, i have added a stored procedure to the linked server which i simply pass the required primary key value to; the Sp on linked server then performs the update.

so use;

exec linkedserver.db.dbo.sp @pk_id

rather than;

update linkedserver.db.dbo.table set a = b were pk_id = @pk_id

Result is no schema lock required and the process is now running in a few seconds rather than 45 minutes.

hope this helps other folk with the same performance hit when using linked servers.

....I would still ike to know if it's possible to prevent the full table scan on linked server table updates, so if anyone can contribute to the thread along these lines please do so!

cheers