Yes, they should turn their siren off, and normally do, personally I always pull forward and to one side providing space, regardless of what the law says I'll do my upmost best to help blue lights through- red light camera sensors don't detect bikes (in my numerous experiences anyway!) and if it was caught on CCTV and they sent me a letter through the post I would happily go to court even knowing that 'legally' I was wrong, do you really think you'd get charged for helping emergency services?
In the past I've even shot up the road when I've seen a choke point to stop a dump truck, giving an ambulance a clear run, but that's just my support riding coming through!