From what I've read over on ClioSport the valves dropping on these engines is usually what does for them (or a belt jumping off). The guys that write all the engine build threads usually seem to discuss replacing the standard valves with uprated ones. They become brittle where they transition from the shaft to the valve seat. People that run maps with overrun (pop and bang map) seem to have issues with dropping values too.
Joking aside, good luck with sourcing a replacement.
As mentioned above, valves dropping does happen on these engines (more so on cars that have been modified yet retained standard valves). The valves tend not to cope well with elevated (above stock) rev limits and modifications that can significantly raise the exhaust gas temperatures. On standard engines it is rare to see failures like this
To err on the side of safety a potential option is to go for something like Supertec valves (which are by far the most common valve upgrade for those modifying or who want that extra bit of security). They aren't cheap but it's possibly worthy of consideration at least.
It says this car has 54K miles which is correct by the MOT history, but another for sale post on here by you had it at 58K miles ? Just wondered if the engine got replaced or rebuilt and if the mileage of 58K is the engine or not ?