I know because I contacted southeastern directly who informed me that it had nothing to do with them and the reference code that was on the letter was formatted incorrectly. The letter asked for my occupation, national insurance number etc. None of which are relevant to an assault investigation.

This letter that the OP was sent was in fact an attempt to steal his information. I received an almost identical one from Southeastern rail today only mine was in relation to an assault that I had apparently commited on a train. The wording was almost identical and the fax number was exactly the same too. I do not travel on trains so wondered what it was all about. I contacted Southeastern and the British transport police, neither of which knew what the letter referenced to! My advice is to always check these things before EVER giving ANYONE any personal information. It is simply unfortunate that the OP happened to travel on trains regularly and had done the said offence so thought it was legit. I do hope he hasn't ended up giving out any personal details!