National Grid has said that:
The total length of the tunnels is 32km and tunnelling was completed in March 2015.
19.6km of the network was constructed using a 3m internal diameter tunnel boring machine
Willesden to St John’s Wood 3m diameter tunnel – 7.4km
Wimbledon to Kensal Green 3m diameter tunnel – 12.2km.
12.4km of the network was built using a 4m internal diameter tunnel boring machine
- St John’s Wood to Hackney 4m diameter tunnel.