Setting log level to 7 has thrown some more light onto this. It would appear that the later kernels don't like either of my sd cards (one of them being the SanDisk Ultra II 2GB 15MB/s that should be 'pi compatible'). The error during boot up is:
mmcblk0: error -84 transferring data, sector 195313, nr 8, cmd response 0x900, card status 0xc00
and on the other card:
mmcblk0: error -84 transferring data, sector 197265, nr 8, cmd response 0x900, card status 0xd00
I guess I've just been unlucky here with my cards here. What I really don't understand is why the 18.104.22.168+ kernel boots without any problems.