Replication stalling on one subscriber
Finally traced the issue to a table on the Subscriber that was missing the PK index. Still not too sure how this could have happened as all subscribers were initialized from the same snapshot at the same time. Re-initializing the individual subscriber had no affect. 
I had to; 
    - remove the article from the publication
    - run the Snapshot Agent
    - manually drop the table on each Subscriber
    - add the article back to the publication
    - re-run the Snapshot Agent

All good now.