You are partly right. The only time a controller does not have any responsibility for separation on a taxiway or manouevering area, is when he or she can't see the aircraft, in which case the taxi clearance will have a clearance limit or a caution attached to it.
In any case, my point is not to apportion blame to a controller, rather to NOT apportion blame to a pilot.
Your marshalling example is spurious as well. A marshaller has little training and does not issue clearances. More to the point, if you are being marshalled onto stand, your protection is in the white lines to either side of you that ensure your clearance from fixed objects.
Now, if we are sitting in our 767, and a vehicle that we can't see drives into the way of the wingtip that we also can't see, and we hit it... how can you possibly lay the blame with the pilot, who was unable to see the problem? You can't stop and ask for assistance if you are unaware of the problem, and if the marshaller doesn't advise you.
In this case, marshallers are often held responsible in that they lose their jobs on the spot.
If you want to see an example of controllers ensuring separation, look no further than LVP's. That is a clear illustration of where responsibility lies when on the taxiway.
Sir George Cayley
At many airfields, aircraft are expected to hold in a "block", which has a single yellow line to signify the rear of the block. Holding in the block ensures your clearance. I don't know if Manchester uses these, but if not, they should.
I agree with the swiss cheese analogy, because if the 737 had been at the holding point, it wouldn't have happened... and if the controller had KNOWN that the 737 was holding a way back from the hold point (which is hard to understand), he or she would no doubt have passed a "caution the 737 holding" advisory... and so on and so forth.