I have just bought a 15 year old 1.4 petrol Fabia so, assuming the timing belts were changed after five and ten years, they are due changing again now. I will arrange for my son-in-law, a mechanic, to replace them for me but if a belt fails does it destroy the engine or does it just bring the engine to a sudden stop, i.e. do the valves encroach into the swept volume or not?

Pretty certain it's an interference engine so yes valves encroach into the swept volume. 

 

As far as I can remember, in recent vehicles only a Mk1 or 2 MX5 is non I t3ference? 

I've never heard of a timing belt failure on a 1.4 16V Fabia, not even one in the twelve years I've been a regular on here.

 

I inspected mine at 125k and it looked like new, as for the small belt that connects the two cams, it doesn't even stretch. The reason this engine doesn't put any strain on the belts is that it uses roller rockers to lift the valves.

 

I'm not saying don't change it, I'm saying that I seriously doubt it's necessary unless it's suffered oil contamination.

OTOH it doesn't need to be a belt breakage that is the actual issue; you can have a failure of the timing belt tensioner with similar effects but an intact belt.