By 1975 the fleet was already many millions of miles into its career and the locomotives, most now in their fourteenth year, were showing their age and requiring more attention and investment to keep them running. The planned electrification of the ECML had not occurred and instead a new HST fleet was on order to take over the Deltic's duties, commencing running in 1978. 1986 was the projected life expectancy of the Deltics and thus a further eleven years were required from the Deltic fleet and as a consequence when 55016 arrived for its Intermediate overhaul in July of 1975 the overhaul program was up-graded and re-designated to Heavy General Repair (HGR) to incorporate the total re-skinning of the interior and the rewiring of each locomotive as well as the removal of virtual every component for overhaul and refitting. The cost of this repair, however, was colossal and by the time the next locomotive arrived in the program, 55019, changes and cut-backs to the level of repair had already been made. Only five Deltics received a HGR and on each the cost and depth of overhaul was curtailed still further until the program was scrapped in late 1976 and Intermediate Repairs were given only, commencing with 55010 in September of that year. To ascertain whether the class could still reach the designated 1986 cut-off 55015 was selected as guinea pig to see if the class could run longer between repairs and as a consequence ran from October 1975 until March 1979 without a classified repair. Again the length of time between overhauls resulted in high costs to put 55015 back into service in May 1979 and as a result the Deltic life expectancy was re-evaluated and shortened from 1986 until the end of 1981.
Here 55016 (D9016) "GORDON HIGHLANDER" is well into its HGR on Sunday 7th March 1976. The loco had arrived for overhaul on the 5th July 1975 and although reports exist that the loco was re-skinned both externally and internally it would appear little replacement of exterior body panels was undertaken. The sanding equipment has been removed and the small access doors have been welded up, yet to be plated over is the route indicator box, maybe this was a late addition to the HGR program - 55016 being the first of the class so treated. Costs even saw this alteration abandoned by 1978 with locos reappearing from overhaul with the glass still in situ. 55016 was released back into traffic following its HGR on the 27th April 1976.