How long does it take to examine my parcel !?

[Snaggletooth]

You might find that you get better customer service in general if you don't level unwarranted criticism at people who are trying to help you.

I think it's pretty clear that you don't have a valid complaint against Royal Mail but the man who sent the parcel might have a valid complaint against the Post Office. The sender should take this up with Post Office customer services on 03457 22 33 44. They need to deal with the chap himself, not you, because it will all revolve around the exact detail of the conversation he had with the clerk at the PO. If you look at the Royal Mail website section on prohibited goods they say this about lighters:

"Lighters and refills containing flammable liquid or gas, including used butane, petrol cigar and cigarette lighters [are prohibited].

New and unused empty lighters may be sent unopened in their original retail packaging."

So you could send, say, an unused Zippo in original packaging etc, but not much else.

Now you said:

.. I told him I had no lighters...

… which sort of implies that the chap sent disposable lighters to you, which would not be allowed, because they contain fuel. So whether it's the Post Office clerk or the sender who is at fault depends on if the clerk passed on the full details about which lighters are and aren't allowed (it's not a straight forward allowed nor not allowed) and whether the sender gave the correct declaration to the clerk. If the clerk got it wrong then the Post Office need to know, because that's a serious training issue. But if the sender gave incorrect information he should keep his mouth shut, because that would mean that he has committed a criminal offence.

The Post Office clerk should have shown the sender a laminated A4 sheet with a summary of restricted and prohibited goods, this does mention flammable substances (e.g. lighter fuel). If done properly this is checkable and would provide some strong circumstantial evidence as to which party is at fault. I think it also shows up on the customer's cash receipt (if he as one), I forget the exact wording but it starts with the letters "DG" and it has a value of 0.00 in the cash column on the right. If that is present on the cash receipt, he doesn't have a leg to stand on. If it's not there, a complaint to the Post Office would be in order.