[Boxybutgood]

To follow up some comments here;

1; The temp gauge has never suggested the car overheated and it didn't lose any coolant
2; The MD has admitted liability for the failed thermostat which they fitted, which was a Subaru item
3; What's wrong with a good s/h head? It won't save me much (in the context of the total spend) and of course if there any more problems it will be down to me as I supplied it. I am also now wondering if replacing just one head is a good idea!

I believe the sequence of events is a failed thermostat and the cracks appearing as a consequence of localised overheating. I realise I can't prove this but it is more likely to me than two catastrophic events happening simultaneously independently of each other.