"The diagram does not tell you to connect the supply as you have stated, and if you are so incompetent that you think it does then there is no way you should be doing the work" is a pure statement of fact in a neutral tone.
It contains not one word of abuse or insult.
I have just looked up "incompetent" in my Microsoft Thesaurus and I would interpret your reply as rude, even if it was intended as a statement of fact.
I read this Forum quite regularly and have learned quite a lot from other peoples experiences but I am being put off by some peoples arrogance and 'know it all' attitude.
Lets all try and be polite even if we are trying to point out to people Health & Safety and Legal obligations.
For an example of how to be polite while addressing the issue of Part P refer to the reply from Chris5