Multi monitor mirrored instead of using primary/selected monitor

When lightdm starts after rebooting or switching on, I'm greeted with the login which is mirrored on both of my displays and the primary monitor resolution is reduced from widescreen to a 4:3 resolution. If I replug the external monitor, the display is no longer mirrored and the correct resolution is displayed on the primary monitor. I also get the Ubuntu logo in the centre of the external display of which I assume is the correct behaviour. This is on a Lenovo G570 notebook.

$ sudo lspci -vv
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09) (prog-if 00 [VGA controller])
 Subsystem: Lenovo Device 3975
 Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
 Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
 Latency: 0
 Interrupt: pin A routed to IRQ 41
 Region 0: Memory at d0000000 (64-bit, non-prefetchable) [size=4M]
 Region 2: Memory at c0000000 (64-bit, prefetchable) [size=256M]
 Region 4: I/O ports at 3000 [size=64]
 Expansion ROM at <unassigned> [disabled]
 Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
  Address: fee0c00c Data: 4161
 Capabilities: [d0] Power Management version 2
  Flags: PMEClk- DSI+ D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
  Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=0 PME-
 Capabilities: [a4] PCI Advanced Features
  AFCap: TP+ FLR+
  AFCtrl: FLR-
  AFStatus: TP-
 Kernel driver in use: i915
 Kernel modules: i915

 $ lsb_release -rd
Description: Ubuntu precise (development branch)
Release: 12.04

$ apt-cache policy lightdm
lightdm:
  Installed: 1.1.8-0ubuntu1
  Candidate: 1.1.8-0ubuntu1
  Version table:
 *** 1.1.8-0ubuntu1 0
        500 http://gb.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status