Neil,
As you've successfully upgrade to the latest firmware version, can you confirm whether you are able to enable IGMP Snooping?
When I last tried to upgrade, I think I received the same error you reported, but eventually managed to get it to upgrade. However, having tried with both 1.10.11 as well as 1.10.12, with either of these versions I am not able to enable IGMP Snooping. I had to downgrade back to 1.00.19, and I've been there since, as that is the last version with which I am able to enable IGMP Snooping.
Thanks,
Corey