Write to the Chief Constable of the force, heading your letter 'Negligence by Investigating Officers'. (Get the address from the force's website)
Word your letter roughly as follows. (Keep it short. You can provide further information later if asked to do so):
"Sir
I reported multiple offences of rape against me to officers of your force last year, naming the offender and directing those officers to evidence in the form of Twitter messages. I was told that a file was to be sent to the Crown Prosecution Service but then told that no further action was to be taken. I assumed that this was because the CPS had decided that there was insufficient evidence for a successful prosecution but I have now found out that the CPS never even received the file. I therefore believe that your officers have been
negligent by failing to submit the evidence to the CPS for their consideration and simply closing the case of their own volition.
I request that an investigation be carried out into this matter and that a case file be presented to the CPS.
Yours . . . "
Send copies (marked 'Copy for information') to:
Independent Police Complaints Commission
PO Box 473
Sale
M33 0BW
and to your local Police and Crime Commissioner. (Get the address from here:
http://apccs.police.uk/find-your-pcc/ )
[Type "Copies to IPCC & _________ (area) PCC" at the bottom of your letter]