Well after a great deal of searching and reading over the weekend, it appears there is no hard and fast rule on this one.
A lot depends (as has been said) as to how much the rear element extends past the recess on the adapter ring when focussed to infinity.
Unfortunately, mine stepped back too much to be clear but, with a little modification (and a LOT of patience) you can add shims between the mount and the lens to push the whole assembly away from the mirror.
You have to experiment a bit with the thickness to make sure that you don't end up with the lens upside down when mounted (did this too many times to count!) but, with a little perseverance and logic, it is possible.
Anyone looking to try it, just make sure the small metal pin on the back of the lens is depressed by one of the shims or you will lose the ability to choose between manual and auto (for those lenses that have it) on the aperture ring.
In short, you CAN safely mount a 50mm f1.4 SMC Takumar on a 5DMkII
(All I need to do now is use the damn thing
)