I have been requested to write a report about what happened on [the] 29th [of] August (or August 29th, this may be American convention) at Heathrow airport. First of all[,] I would like to highlight that I do not support drug dealers, as I am concerned about the problems that drugs are currently producing[causing] in our society.
On 29th August (consider changing as above) I was travelling from Madrid to London by plane with a friend, Dave Wesley. While we were checking in[,] a young boy started a conversation with my friend. The stranger told Dave about the difference of prices between both countries and recognized(admitted?) that he was carrying some cigarettes. Quoting the stranger: Ď["]They only let you carry 200 duty-free cigarettes in. Iíve taken a risk, Iíve got 400. But if they catch me at C[c]ustoms Iíll have to pay a fine, and I couldnít afford thatĒ. (note: originally, a sing le quote ' is used to quote within a quote e.g. "He said 'no'." some books and even reference books have taken to using single quotes to save ink, but then...this is the internet and so we should abandon with the confusing single quote habit. I see you end with double quotes, clearly a typo, but note the rule anyway.)
Taking into account the friendly and generous character of Dave[,] his offer to carry 200 cigarettes himself was not strange(I recognise you may be writing for complexity but this sentence is just too convoluted, "Since Dave is such a generous and friendly person, it was only natural that he offer/offered to carry 200 cigarettes himself"). Then I personally saw how[that] he took a package from the stranger that, eventually[as it turned out], had no cigarettes in[it].
When we arrived to[in/at] London[,] Dave was held up at customs(not captitalised, right?), while I was in a hurry to catch my connection to Oxford and did no[not] realised about anything[what had happened].
The drug dealer measured[was ("measured" makes him sound like a fish)] 1,[.]70 meters more or less; he was bold(?) and was wearing sunglasses. From his appearance he must be[have been] in his late twenties and told us that he was currently studying at Christ Church in Oxford.
Looking for justice
Having reported the whole story I demand[would like to request/strongly request/must request (watch your tone, right?)] that further investigation be done in order to find the real drug dealer.
If you need[require (don't ask what authorities/superiors need from you, but what they require)] any further information[,] please, do not hesitate in[to] contact me either via email or (via)?(remove this) mobile phone. I will be pleased[happy] to tell every detail I remember in order to help and innocent person who is suffering [as] the result of a lie(deception (no "a"), trick, con, or scam might be better words).
--Hope this is useful.