When I last visited Avebury I wanted to re-visit the museum. I reached the ticket point and flashed my CADW card which gives free entry via the E.H. tie-in.
'Oh' said the lady in quite a dismissive tone 'you do realise you are ONLY allowed in the museum and NOT the manor house with THIS'. I thanked her and said I did know and that I only wanted to visit the museum!
'With great power comes great responsibility' - as someone was once told :)