I thought I'd update this thread with my experiences - bear in mind this is in the UK and is something that seems to differ in different countries.

I started by transferring MR points to my husband's FF account (same surname) and all went fine.

So then I tried transferring some MR points from my sister's Amex account (different surname) to my FF account. The Amex website showed my FF account no as having been accepted by the system before I tried the transfer. The points disappeared from her Amex account but never appeared in my FF account but neither did they reappear in her account, as I would have expected had the transfer been rejected. After speaking to Amex, it seems they would have expected the points to have bounced back too so they are currently trying to work out what happened and are redoing the same transfer to my sister's own FF account instead.

Amex mentioned that the transfer failed because of the different surnames, so regardless of the fact that you are not supposed to transfer into a different person's FF account, it seems it will work in the UK as long as you share your last name.