You can agree a price with a private buyer immediately. BUT you can NOT sell to anyone who was shown the property through the estate agents.
Also, if you have an estate agent board up in the front garden, take it down IMMEDIATELY. There is a chance that the estate agents could say that although they were no longer instructed the board was showing that your house was for sale. If, lets say, I walked past the property and called the estate agents, they would have to tell me that the property was no longer on their books. If they found out that you sold it to them they MIGHT have a claim on the 1.25%.
Hope this helps.