Thanks Ferdi,
Regarding my second question, I've read somewhere that the viewing lens on the 3.5f is actually 2.8, although I could be wrong (and I often am).
With the filter holders I'm afraid the 100mm might be my only choice as I can then use it on my digital. I had an 85mm filter system from Formatt Hitech on my Fuji x-e2 and my images were vignetting when using the Samyang 12mm so I feel I just need to go bigger.
You say the hard edge grads will be harder on medium format? I've had it in my head that the hard edge grad will appear softer edged on something like the Rollei's lens as it is quite small in diameter. A soft edge therefore will appear really soft as the grad width would almost cover the lens. Can you explain further?
Thanks again,
Alastair