Basically, the retainers connect the valves to their springs and if the retainers should fail the valves can drop into the engine and more or less write it off. Failure is thought to be started by over-revving (EG changing down to a low gear causing the engine to exceed its safe limit). The retainers then start to crack but fail at a later point in time so you may think you've got away with it but you might not have. From some of the other posts on the subject it seems as if '04 model on retainers have a little bit more material in them and don't suffer in this way, and only the inlet valve retainers on pre '04's are at risk (maybe because the inlet valves have more mass?).

I became worried about this the other week and carried out a check on my car (easy to do):

https://www.s2ki.com/forums/index.ph...&f=25&t=434380