(The latest in this saga. Recap: head cracked - compression burst the radiator - new head just fitted.)
The repair shop have put it all back together and it started first time! Then the noise started... when it's revved, there's a loud and harsh intermittent clatter. Apart from that it runs and pulls fine, with good oil pressure.
The clattering is so dramatic it must be serious but the shop can't figure it - if anyone has any ideas, I'd be pleased to hear them.