The police seized a computer from my home address for the purposes of gathering evidence against me, if I believe there is evidence on the computer that clears me, and the police do not mention it in interview, can I request the computer returned to me, so I can provide the evidence myself?
Unlikely. The police will have taken a byte-by-byte copy of the hard disk(s) in the machine, but will generally speaking not return the machine to you until the CPS instruct them to drop the case, or the case goes to court and you are acquitted. If the case goes to court and you are convicted, they will probably never return the machine to you...
Best thing to do is to get your solicitor to request the file(s) which you believe will prove your innocence...
However, the police have a duty to disclose to you any material which they discover but they do not use. They must provide this as "unused material". You need a solicitor to keep on top of this.
Also .. the Police forensic/scientific computer 'specialist' will write a report that will/could be used as evidence .. anything apertaining will be mentioned in this report. Part of the 'duty' mentioned above will be in detailing connected information. If not connected, you can only make representations to judge through solicitor. Judge may then order retreival.
If they find nothing, you will get it back.If they find something and this is used as evidence against you, the judge will order the computer destroyed.