I've rebooted, reinstalled and tried again but since SourceTree was updated yesterday it now won't launch at all. Here's the crash report:
Process: SourceTree [592]
Path: /Applications/SourceTree.app/Contents/MacOS/SourceTree
Identifier: com.torusknot.SourceTreeNotMAS
Version: 2.0.5.6 (2.0.5.6)
Code Type: X86-64 (Native)
Parent Process: launchd [314]
User ID: 501
Date/Time: 2015-11-18 09:17:35.542 +0000
OS Version: Mac OS X 10.8.5 (12F2560)
Report Version: 10
Interval Since Last Report: 4667 sec
Crashes Since Last Report: 213
Per-App Interval Since Last Report: 132 sec
Per-App Crashes Since Last Report: 6
Anonymous UUID: B34695F2-A3EB-37B1-951A-A26E89592103
Crashed Thread: 8 Dispatch queue: com.apple.root.high-priority
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000038
VM Regions Near 0x38:
-->
__TEXT 0000000109646000-0000000109904000 [ 2808K] r-x/rwx SM=COW /Applications/SourceTree.app/Contents/MacOS/SourceTree
Thread 0:: Dispatch queue: com.apple.main-threadI've just now realised I have version 2.0.5.5 on my Yosemite partition and copied it over so it's all working again but there's clearly a showstopper bug in 2.0.5.6 for Mountain Lion users.
EDIT: I did paste the crash report above but my post was rejected for blacklisted words so...