How is it that the police can destroy evidence?

Ricky Tomlinson holding a copy of Northern Voices at a Conference of the FBU

YESTERDAY, Brian Reade wrote a piece in the Daily Mirror about Ricky Tomlinson's claim that Richard Whiteley was a spy for MI5.  We produce an excerp below:
'Not convinced? Neither is veteran Shrewsbury 24 researcher, Eileen Turnbull, who believes Tomlinson may have been duped: “I don’t know why Ricky is saying this,” she said.
Well, having recently spoken at length with Ricky, I think I know why. The 77-year-old realises he hasn’t got too many fighting years left, and the lack of a pardon for him and his fellow strikers in the face of overwhelming evidence that they were framed, could be driving him to distraction.
In 1973, Ricky was jailed for two years at Shrewsbury Crown Court, and 23 others convicted, after being found guilty of arcane public order offences during a national building strike against poor pay and Victorian working conditions.
For 44 years, campaigners have insisted the convictions were instigated by Ted Heath’s Tory Government who feared the rise of trade union power.
They have documents suggesting police destroyed witness statements and framed testimony to convict the activists, and that crucial papers are being withheld because they are too damaging and embarrassing to reveal.'
The Royle Family starhad said:
'... had he know of his alleged involvement in the plot when he appeared on Countdown he would have throttled him.'
It looks like Ricky is keen to get this issue of the Shrewsbury Pickets out into the public domain. 

Ricky Tomlinson outs Richard Whiteley as MI5 Spy

RICKY Tomlinson, the actor star of The Royle Family reckons Richard Whiteley, the late TV Countdown presenter was working for the security services. 
Ricky made the claim about Whiteley during an interview marking the opening of a Wetherspoons pub in Chester.
Mr. Tomlinson, was a plasterer before taking up a career in comedy, and he helped to organise the controversial national building workers’ strike in during the 70’s at the same pub.
In 1973, he was sentenced to two years in prison after having been found guilty of 'conspiracy to intimidate' as one of the so-called Shrewsbury Two with Des Warren
The actor and other campaigners have long believed that he was the victim of an establishment set-up.
The Labour MP and a former Defence Minister Peter Kilfoyle is now calling on the Government to own up over the affair, which has remained shrouded in secrecy for 35 years.
Mr Kilfoyle spoke out after the Cabinet Office refused to release its secret files on the case, which include a report to the then Prime Minister Edward Heath, because of the need to 'protect the security services'.
Today the Daily Express reports:  'Files released earlier this year show the then head of MI5, Sir Michael Hanley, intervened personally to block Ricky’s release, claiming that he was involved in a communist plot to destabilise Britain.'
Ricky and his friend Dezzie Warren were dubbed the 'Shrewsbury Two' after being jailed for organising a picket in the town in 1972. The pair, who both spent much of their sentences in solitary confinement, staged a 22-day hunger strike in a bid to be declared political prisoners. Mr Kilfoyle said he would now launch an appeal to get the information released.