Many thanks for responses. As I see it, the RPi B+ looks like the best platform for running RISCOS, being the most advanced RPi board still using a single core CPU (all that RISCOS needs). So my main worry is that no one else has reported this problem...
I have unearthed another RPi B+, so I now have four nominally identical boards. Two boards ('earlier') were bought in late 2014, have hardware revision (BCM2708) 0010, and Samsung RAM. The other two ('later') were bought this year, and differ only in having hardware revision 900032, and Elpida RAM.
If I plug an SD card loaded with RISCOS (RC14, current release) and associated files in to the earlier RPi B+ boards, RISCOS runs correctly; with the later RPi B+ boards, I just get the green led flashing in groups of 8 (? SD card not recognised). The RISCOS SD card also works fine on RPi B and 2B boards. Raspbian runs correctly on all the boards.
I have also used NOOBS to load a 16 GB SD card with both RISCOS and Raspbian Jessie. On the earlier RPi B+ cards, I can run either RISCOS or Raspbian. On the later RPI B+ cards, Raspbian runs but RISCOS does not.
Not sure whether this is worth pursuing, as I can work around it by simply swapping cards around, and go and do something more productive; but it would be nice to be able to understand this.