I seemed to have stabilised the system somewhat. I bought an 8GB class 10 microSD card and SD adaptor. Now I see somewhere here that class 10 cards are not necessarily stable, so that might have been a mistake. Anyway, I havnt managed to trash it yet. It would be nice to gain access to all 8GB, as partitioning it before loading the image doesn't work, since the disk image overwrites the partitioning info. I have reformatted an old 256MB USB stick as an Acorn hard drive which is quite useful for backing up applications without losing the Acorn filetypes - I noted that the SparkFS bundled app is only read-only, so cant be used to make proper zipped archives.
I have not tried slowing down SD card access yet - surely it should be OK with a class 10?
I am still get some freezes, but less than with my previous SD card, although I am being a bit careful when my 4GB USB stick is plugged in, since transferring to that seems to often lockup the machine.
I have some custom software used in our university lab which appears not to be fully compatible with the Pi - sometimes freezes, and sometimes runs, and I am having difficulty understanding why.
I just tried to run a copy of Elite downloaded from http://www.iancgbell.clara.net/elite/arc/index.htm
but it reports "CLockUtils is not 32-bit compatible. I tried commenting out the load of that in the MainRun file, but the I get an abort on instruction fetch at &FFFFE94E which is presumably the 26/32 bit problem - I am loathe to run it in the emulator, since that seems to negate the point of having RiscOS on the Pi, but I suppose I will have to try it.