The following extract from the London Canals site seems to confirm the 1970's with additional work in 1985.
During the 1970's a huge scheme to install electricity cables underneath the towpath between Acton power station and Islington got underway. Despite not actually being part of the towpath improvement schemes, it did contribute greatly towards a better environment for walking. By the mid 1970's people were easily able to walk westwards from Little Venice as far as the Grand Junction Arms in Harlesden on a hard surface towpath. Unfortunately this scheme introduced that awful contraption which still stands to this day above the eastern portal of Maida Hill tunnel. This 'contraption' carries the electricity cables down to the towpath after travelling under the pavements between Blomfield Road and Aberdeen Place.
In 1985 the towpath was further improved and widened by the electricity generating board. Much of the canal towpath between Harlseden and East London is its responsibility as well as Canal & River Trust's.