He joined the Communist Party of Great Britain there in 1960 before being recruited with other shop stewards to Gerry Healy's then Socialist Labour League in 1966.
[1] However, in 1974 he and most of the Cowley group were expelled - from what had become the Workers Revolutionary Party the previous year - with around 200 other members.
Around a hundred of them went on to form the Workers Socialist League (WSL) of which Thornett was a leader.
Political differences emerged in the new organisation with parts of the ex-WSL splitting off before those remaining were expelled in 1984.
Thornett successfully argued for the ISG to exit the Labour Party and join the Socialist Alliance.