Could you narrow the parameters of the question?
"...Is their a code of conduct for a neighbourhood plan..?"
No, not necessarily but possibly yes. There are guidelines for the development of a neighbourhood plan but, is the neighbourhood plan being devised / implemented by a PC or an otherwise constituted committee?
There is a code of conduct for individual councillors but not for a PC.
"...Are they allowed to contact an estate agent emailing their concerns about land that has been allocated on the local plan..."
Who is they?
"...They have sent a very aggressive email stating absolute untruths about their feelings of the land on the local plan detailing their objections..."
Again, Who is they?
There are elements of common and planning law which allow for compensation for adverse [lets say] "interference" - but it may depend who "they" are.
"...We have fully cooperated with the local council who are happy with our pre application..."
So is this an accurate assumption - you are a planning applicant, either a PC or an other group/committee has some objection to your application, there is either a draft or adopted NDP in effect and the party yet to be determined has contacted an estate agent with an interest in a plot of land subject to your application and they have interjected with an adverse contribution?
"...I feel this is a personal attack and feel we should put a formal complaint in as this is not how a neighbourhood plan should conduct them selves..."
A "neighbourhood plan" isn't really the right way to describe an "entity" - whatever that entity might be. You need to determine what it is you want to complain about and who.
If what you mean is a PC is developing a neighbourhood plan then the complaint might be against the PC - bit that has its own difficulties anyway.
Would it be possible to be a bit more specific about the details and who / what you feel has wronged you...