Complex sentence: In 1970, the GLC estimated for the Treasury that the cost of building just Ringway 1 was £1.7billion (about £18billion in 2006) three-quarters of which would have to come from central government grants.
Split into simpler sentences: In 1970, the GLC estimated for the Treasury that the cost of building just Ringway 1 was £1.7 billion (about £18 billion in 2006). Three-quarters of this would have to come from central government grants.