Probably too late but future use if you are checking this or asked for flagged alert to answer.
THIS RELATES TO UNITED KINGDOM LAW.
It's simple NO they can't demand entry you CAN keep them out. The only ones you can't are a balif and or police but they must have a court order.
All others go by the old saying
'IF IN DOUBT KEEP EM OUT' close the door and make enquiries to their company through the number in the directory or on a bill. NEVER from a number they may offer, it might be genuine but it could be bogus.
Nobby
January 2009