An FI can give training from an unlicensed aerodrome any time he likes and that includes a trial lesson.
I suggest you read the latest ANO:
Flights which must use licensed or Government aerodrome
208.—(1) Subject to paragraph (5), article 207 applies to any aeroplane which has a maximum total weight authorised of more than 2730kg flying on a flight—
(b) for the purpose of instruction in flying given to any person for the purpose of becoming qualified for the grant of a pilot’s licence or the inclusion of an aircraft rating, a night rating or a night qualification in a licence;
Now a trial lesson can only be for the purpose of training for a licence; if not, it is either a Public Transport flight or a Private Flight. If it is a private flight the passenger may only pay a proportion of the direct operating costs!
A FI is not confined to only giving instruction at a RTF or FTO; these are JAA licensing requirements and do not apply to the NPPL.