He joined the Transport and General Workers' Union (TGWU), becoming prominent in its Cab Section.
By the end of the 1970s, he was Chair of the Region 1 Cab Trade Committee, and in 1980 he was elected to the union's General Executive Council.
In this role, he devised an index which was later adopted by the Department of Transport to calculate annual increases in taxi fares.
[1][2] During his time at the T&G he also wrote a document called a National Framework for Taxis, which was referred to in the parliamentary debate around the Private Hire Vehicles (London) Act 1998.
[1][2] In his obituary, Barry Camfield described Hagger as "the most influential lay trade-union activist in Britain".