In the 1970s the police were renowned for “fitting up” criminals whom they had very good grounds for believing had committed serious offences but where they were short of evidence that would secure a conviction. Perhaps the most famous (or infamous) case where this was possibly true was that of George Davis. I lived in East London at that time and “knew people who knew people” (including a serving police officer who was subsequently jailed for perjury in an unrelated matter) who led me to believe that Mr Davis had indeed been fitted up. The police were quite certain that he had been involved in serious crimes but it seems unlikely to me that he was guilty of the offence for which he was convicted.
In 1976 PACE (the Police and Criminal Evidence Bill) was introduced and this placed strict rules on police procedures and prosecution practices. It curtailed the ability of police to fit people up. However, developments this week (if they are found to be true) demonstrate that it is not possible to entirely remove the risk of wrongful convictions. Nothing can be done to prevent people - even police officers - lying in statements to the police or later in court.
The worrying thing about the police at present is, as you mention Gromit, misbehaviour (ranging from minor misdemeanours up to serious criminal offences) seems endemic across the ranks up to and including Chief Constables. It is not surprising therefore that some rank and file officers behave in the way that they do. Scarcely a month passes without a story or two of wrongdoing by police officers coming out.
If officers have lied to an extent that led to the downfall of a senior politician it is very worrying indeed. These officers, though not senior in rank, undertook extremely sensitive and responsible duties and so were, to a degree, a cut above the usual rank and file. If they have fabricated evidence they have let down many thousands of their honest colleagues who may find it increasingly difficult to have their evidence accepted by magistrates and juries. And then we’re all losers.