The first oversight is the accuracy of the information. For example, when the MI5 officer Michael Smith was prosecuted under the Official Secrets Act for passing on confidential data, MI5 admitted that he had been cleared to work for the agency because details of his Communist sympathies had been placed on the file of another Michael Smith.
The Data Protection Registrar, has persistently stated that MI5 - together with MI6 and GCHQ - should register under the Data Protection Act 1984. This would require that all data is "collected fairly and lawfully, is accurate and kept up to date, and only used for the purpose for which it was gathered". But MI5 has refused to comply.
There is considerable evidence that MI5 acted to undermine a democratically elected Labour government in the 1970s. Political figures of all the main parties were subject to MI5-inspired dirty tricks campaigns.
The first move should be to establish an independent commission to investigate the accuracy and legality of the data, and any complicity on the part of MI5 officers.
If there had been some 290,000 subversives, wouldn't there have been some obvious evidence of this vast revolutionary army in recent decades?