Any question answered...

Are IS mounts specific to the size of disc rotor, or is that governed by the adapter which you use?

If so, who can find an SM-MA90-R140P/S for sale from a reputable UK shop?

Pretty sure it's all in the adapter.

Depends on what the IS mount was initially specced for I'd have thought. My Wound-Up is IS 160 - I'd be surprised if I could inverse shim it to 140.

Is this for the Oak? Rear brake mount? Check the spec on the drop-out.