So turns out that the cabin pollen filter for an MG zs is not the same as for an M... It's much bigger. Even bigger than the plastic tray that the oem one fits in.
Every time I've changed the filter, the one I've got has had to go in instead of the tray, rather than in it, and since the honda one isn't available (at least on lings), I thought I'd give the MG one a bash.
Got one from euro car parts instead (still didn't fit in the tray though) and hoovered out years and years of leaves and crap. Fans sound much happier now