Posting this from the rPi using the default Midori browser. I bought a new SD card today - I went into Maplins and asked them for the cheapest card they had. The rPi is working from it now. It's a CnMemory (no, I never heard of them either) 8GB Class 4 card. I didn't see any error messages during the boot phase and the rPi definitely seems to run quicker than with the micro SD card plus adapter.
For a 'lightweight desktop' with a 'lightweight browser' it seems pretty good. Some of the bigger animated smilies and avatars flicker, but other than that it seems pretty much like firefox.