I did a Google as my memory on these engines is scarce at best, one quote said it's an interference engine as components get very squishy close and if something pops it's bye bye valves !

Many thanks Marty; good reason to treat them with respect by changing the oil every 12 months or 5000k, strictly WHICHEVER COMES FIRST, which will be 12 months for the majority of Classics

Interference motor ('tho heads had been machined at least once....).
I had a crankshaft gear lose a tooth, that  then munched the fibre camshaft gear (i.e. stopped rotating), which meant valves were down when should have been up.  Yup, at least one bent valve but fortunately no holes in pistons.

Thanks everyone for great info