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Turns out it was the lens that was the problem! This is the dinky 40 2.8 so I'll go down a youtube rabbit hole of learning to service vintage lenses...
Turkish-
It's not that hard with the right tools. Just with a lens spanner I've managed to get a few lenses apart to be able to clean haze (it's always the element behind the aperture blades).
This is pretty good for Pentax SMC-M lenses
https://youtu.be/pjYoDIpw_Jw
Ste_S
As @supersalad expertly demonstrated, it could be either. Should be easy to find out which one though by working the levers manually, both should snap back instantly. Should be a fair bit of resistance on the body lever also.
I'd still guess at it being gunk on the aperture blades in the lens causing problems, with perhaps the MV body lever having a bit more snap on stop down.