MI5 has a record of targeting left-wing MPs and parties in Britain, sometimes via retired agents and third parties. The best known examples are the ‘Zinoviev Letter’ affair of 1924, and the ‘Wilson plot’ (that is, the anti-Wilson plot) of 1974-6. (See my Plots and Paranoia, 1989.) So I wouldn’t reject Len McCluskey’s suspicions out of hand.
What usually happens in these cases is that the charges are dismissed by the authorities at the time as the crazed ravings of ‘conspiracy theorists’. No-one wants to be labelled a conspiracy theorist – and there are, in truth, some real nutters among them, especially on the web – so respectable people are reluctant to countenance such claims. Then, thirty years later, when the archives are opened, the suspicions are shown to have been pretty well justified; by which time, of course, it’s too late.