Shambles: re. your question .. yes, exactly what is an authorised person?

Currently only Customs and Excise can enter a premises without a warrant ...the Police can enter a premises with a warrant

I'd missed this point .. new powers to authorise PEL officers?

What is exactly "reasonable" force?

I was left black and blue after a sub-Postmaster used "reasonable" force to exact a citizen's arrest and to throw me to the ground and drag me along by my wrists when I left his shop after a minor row (about a bloody postcard!)
NB: it turned out it wasn't the first time he'd been spoken to the Police about his "attitude" - he also regularly verbally abuses the pensioners who go into the sub-POst Office - altho' on a purely technical point (he "believed" I was leaving the shop with a 20p postcard!) he was entitled to make a Citizen's arrest.

So, "believing" an offence might be taking place gives carte blanche to beat up small women!