Problem with Sophos Rescue disk scan freezing after update of engine to 3.75

I've created a Sophos Rescue disk (SBAV) to run a scan on a standalone (non networked) laptop and the scan freezes after a while. 

I've tried it on two different laptops with the same result - the time taken to freeze varies between PCs but on individual laptops it always freezes at roughly the same time. 

After booting from the disk, the scan progresses normally, showing filenames as it scans, then freezes showing a message where the name of the file being scanned would normally be in the format:

PHYSICAL:0000:0000:0000:0000 (that's the format - I've not made a note of the actual numbers).

I started a scan yesterday evening and left it running, in the morning it was stuck at that point. 

Previous versions running the 3.74 engine work fine.

I've run chkdsk with no errors reported and also run a defrag on the first PC this happened on (it's an oldish XP machine). I've subsequently run the scan on a newer Dell Laptop running Windows 7, with the same result.

Any ideas?

Chris