Many thanks everyone for your advice and suggestions.
I my initial thoughts were that Ancestry didn't allow people using free access to contact other users as a way of getting them to pay up, but on reflection perhaps it is to protect the paid-up members from getting contacts from people who won't be able to communicate further once the free time is over.
To help other Rootschatters it might be an idea to have some 'official' guidance somewhere on the site? If requests are allowed then the point about people making corrections not necessarily being related to the person is also something important to make people aware of.
Nick