Correct I did miss that out, mostly because its not relevent, the question was what power to detain is there? The power to detain is S. 89(1) as stated.
In actual fact there may be a second power which is contained under the power to establish cordons and prevent people fom crossing them. I think that may have been stretching things a little far!
But yes your assessment is correct you may not be guilty of an offence but by not complying with this act you could become guilty. If you think about it though if you are directed to stop your car by a constable acting under the Road Traffic Act, you may not be guilty of any offence, but if you fail to stop you become so. Same with giving name and address under the same act...the list could go on, so this is nothing new.