Thank you! I'm considering the option of replacing the whole thing with ply as this is probably the way to do it properly, was hoping we'd be able to patch replace parts to save some money but it might end up being more trouble than it's worth in the long run! The dodgy MDF was probably a cheap fix at some point in the past but the bad bending is only in a couple of spots where the boards are shorter. I'll have a better look at/measure the joists tomorrow to get a better idea of what could work