I would say that the police are wrong.
It is totally and utterly your husbands responsibility to check and double check before making any move.
I would say that the accident is your husbands fault.
Even if he did signal and check his mirrors he still did not see the motorcyclist so therefore did not check properly did he.
Your husband is 100% to blame , the MC was filtering outside slow moving or stationary traffic which is perfectly legal , your husband didn't check his blind spot , he is to blame .
It is the responsibility of the driver changing lane or turning to ensure it is safe to do so , your husband is 100% to blame .
I suggest you both read the highway code .
You're going to lose so I suggest you settle out of court , it will be cheaper .
Rule 88
http://www.direct.gov.uk/en/TravelAndTransport/Highwaycode/DG_069854
Frankly your husband should have been done for driving without due care and attention , the coppers who attended should be sacked for their negligent .