ubuntu11.10 fails to boot after update

I happily installed ubuntu 11.10 on my Acer Aspire One netbook some weeks ago and have been happily working since then. A couple of days ago I was suddenly unable to boot into X. I hadn't made any changes that could explain the sudden failure as far as I am aware. The boot process seems to hang attempting to start LightDM (reported as "fail" in the boot output).

On further investigation I think X has failed to initialise the fglrx driver properly:
matt@matt-netbook:~$ tail -30 Xorg.0.log
[ 113.947] (II) Unloading radeon
[ 113.947] (II) UnloadModule: "vesa"
[ 113.947] (II) Unloading vesa
[ 113.947] (II) UnloadModule: "fbdev"
[ 113.947] (II) Unloading fbdev
[ 113.947] (II) UnloadModule: "fbdevhw"
[ 113.947] (II) Unloading fbdevhw
[ 113.947] (--) Depth 24 pixmap format is 32 bpp
[ 113.947] (EE) fglrx(0): atiddxDriScreenInit failed, GPS not been initialized.
[ 113.947] (WW) fglrx(0): ***********************************************************
[ 113.947] (WW) fglrx(0): * DRI initialization failed *
[ 113.947] (WW) fglrx(0): * kernel module (fglrx.ko) may be missing or incompatible *
[ 113.947] (WW) fglrx(0): * 2D and 3D acceleration disabled *
[ 113.947] (WW) fglrx(0): ***********************************************************
[ 113.947] (II) fglrx(0): FBADPhys: 0xc2900000 FBMappedSize: 0x10000000
[ 113.947] (II) fglrx(0): Reserved 0x02900000 bytes of sideport memory for power saving
[ 113.947] (EE) fglrx(0): Failed to map FB memory
[ 113.947] (II) fglrx(0): === [xdl_xs110_atiddxScreenInit] === end
[ 113.947]
Fatal server error:
[ 113.947] AddScreen/ScreenInit failed for driver 0
[ 113.947]
[ 113.947]
Please consult the The X.Org Foundation support
  at http://wiki.x.org
 for help.
[ 113.947] Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[ 113.947]
[ 114.346] (EE) fglrx(0): firegl_SetSuspendResumeState FAILED -9.
[ 114.501] ddxSigGiveUp: Closing log

lsmod | grep fglrx returns no results

The default kernel version in GRUB is 3.0.0-13. I have discovered that by selecting a previous version (3.0.0-12) I am able to boot successfully.

lsmod | grep fglrx on 3.0.0-12 now returns:
fglrx 2929009 52

The date stamp on 3.0.0-13 in /boot is beginning of November and I have successfully booted since then, so I suspect some subsequent automatic update has killed my kernel config (at least I haven't knowingly modified any kernel setup).

Help! This is where my knowledge runs out. How do I fix my kernel to boot correctly without having to manually drop back to 3.0.0-12 all the time?

Thanks!!