From reading the guides, the U20 is aparently the most common chip to go.  From looking it doesn't look to have been replaced (not socketed) but the LM339 has.  I reseated it but no luck. The machine I don't believe has had a hugely hard life so it may have slipped through.  I thought the clunking of the playfield on those damm hinges this morning I might have pinched something, but looked and looked and could see nothing.  I tested the pin 3 (connector side) on buzz to earth and nothing.  From googling it seems that the ground short error is never likely to be an actual ground short, but a 'something broke' error!  More coincidence I think.  But happy to get it out of the way now, the power board has been done, everything else clean.

The board will be out for a week as I won't be around (working up Marysville way)....