Why don't you just forget about it and have a pint and a chill out instead?
This is the best advice. If you suffered a financial loss because of the estate agents direct actions there would be strong motivation to pursuit a complaint. But complaining to a company who you didn't employ about the service they gave you sounds like a waste of time. Sometimes it is worthwhile to complain so if you feel strongly and have a written log of convos etc then ok. Otherwise I'd just crack on and enjoy your new home without letting them take up another moment of your time. Insincere letter back saying thanks for your feedback and we are sorry is hardly worth the hassle imo.
pretty much this.
what kind of outcome are you hoping to achieve from the complaint - an apology? probably not worth the time and effort and they won't mean it anyway.
if you just want to vent and let off some steam in their general direction then it doesn't really matter how you structure it - just write what you like and get it sent off if it will make you feel a bit better.
Sounds like the whole moving thing was a pretty stressful experience, looking forward and being happy that you don't have to deal with them anymore ftw.