Have employed an estate agent but I may not need them - where do I stand

oldbikebloke

your contract almost certainly is water tight where the estate agent has "introduced" your buyer to you
whether your contract clearly defines what "introduced" means can be established by reading it 

if the neighbour never set foot in the EA, and the EA has no record of their existence, then the fact the neighbour saw a board may, or may not, stand up in court as an "introduction".
One thing for certain, the EA will know the instant the property is sold and will be going through its records with a fine tooth comb to see if they had any contact at all with your purchaser and therefore did "introduce" and can claim a fee

whether your contract entitles them to a fee in other circumstances can be established by reading it