Just to share my experience of the plastic bailers. I’ve had a Mk 4 since 2007. Replaced both bailers in about 2010 with identical plastic ones because the ‘flaps’ failed on both. Easy to replace but not cheap. One of the replacements has subsequently failed when the hinge at the front gave up, thus letting water flow in rather than out. This was despite being careful about ensuring the self bailers were closed when on the trolley. I’ve now replaced that one with a stainless steel one. There was a fair amount of effort and ‘formwork’ and epoxy and gel coat and sanding required to make the aperture in the hull 2mm or so smaller and also to fill the bolt holes ready for drilling new ones in marginally different places. The boat is at Notts County Sailing Club so available for inspection at the UKWA cruising conference in March if you want a closer look, not that there is really anything much to see!
In relation to sticking flaps, prior to the plastic ones breaking I experienced the flaps sticking frequently. My solution was to permanently remove the grill, and one could then gently poke the flap open from within the boat if ever it stuck.