If it's anything like the undertray fixings for my 159, the problem will have been the original seizing (with rust) in their mounting points. So some of them simply shear off. Others, as the mounting points are nut-serts (a threaded rivet basically - allows a full thread in a thin wall box section), the nut-sert spins in the subframe. So the only way to remove the undertay is force it past, or cut the screw.
Due to other issues, I took advantage of having the engine out to drop the subframe. I've welded all my nut-serts to prevent them rotating in the future, replaced two that had sheared internally, and managed to release two seized screws (due to the heat from welding the nut-sert).
If you are luckier than me, then you've simply got missing screws - and as they are standard M6 thread, some decent plaed M6 hex-head screws,say 20mm long, and some penny washers will do fine. I'm intending using stainless screws, simply because I've got them, and they aren't exactly going to be over-stressed holding on the undertray. Just coat the screw threads in anti-seize compound of some form to allow them some chance of coming out again.
Just seen your location - if you're anywhere near Skipton/Keighley, come take a look.