I think The timing is about right. I decided to romove the rocker cover to see if I could see any evidence of valve damage. The inlet rocker shaft was so loose the rockers on cyl 1 had about 20mm clearance!
We threadlocked and re-torqed all the rocker bolts and are in the process of replacing enough components to fire it up again - fingers crossed!
There was no evidence of damage to the threads so I'm hoping that all this wasn't the result of piston/valve contact.