The only way disposal of the tape could affect the outcome of a trial would be if it had been requested as evidence, and then it was recycled, which could be viewed as trying to prejudice the judicial process. Remember that EVERY person coming into custody at a designated custody suite is videoed, and it would be totally impractical to keep all tapes until all the cases have been dealt with. Some cases could take months, or even years (for crown court) to resolve.
What happens is if there is a complaint made by a person in custody, or is something significant happens in terms of the prosecution case, then the recording is preserved in case it is needed. It is for the defence to request a copy of the tape, if they require it, but that window of opportunity is not there after about 28 days. Solicitors often request the recording be preserved in case they need it, rather than because they need it, but with no request made.... bye bye recording.
They have to keep the actual custody record for years, (this records personal details, visits and other procedural matters, but only what the custody officer records on the computer) and this can be requested for up to 12 months after your period of detention. This may well say about the offer of a further sample, and andy mac saying he didn't want to, but he agrees that this is what happened. What he needs to find is the actual words used in the offer, and that could come from the recording. (although not all of the custody suite may be covered, some areas may have just monitoring cameras with no recording, with just the custody desk being constantly recorded)
Andy mac could write to the police, saying that in his view he was persuaded by the officer to not provide an alternative sample, and asking for the custody video to be preserved as evidence in order to be able to see what the officer said. Then he would have a case for a not guilty result if they recycled it, because he has indicated that he views it as evidence. But he is then showing his hand too early in the proceedings and might 'assist' the officer when he is writing his statement. All he has to do is write saying: Name, date of birth, address. I was taken into custody at XXX police station at about xxxx hrs on DATE. I request that you supply me with a copy of my custody record and a copy of the custody video relating to my time in custody.
Once he has viewed the custody video he will know if what the officer is said to have told him about not bothering with further samples is on record. If it is... a win is assured.It will also help remind him what he said.... so that his recollection in court does not prove to be at odds with what is on the video, and the police use it as evidence themselves.
Just in case I am only quoting local policy, I have had a quick search for guidelines on the keeping of custody recordings. West Mercia and Thames Valley both quote 28 days. The Met, however, say 12 months on division, then 7 years at the central archive, so there is a variance force by force.