I have gone for Summer 1983 for my 'official' timetable as the there were still a lot of 40s going up and down the coast then. That was also when the services to Scarborough started up which brought the class 45's across more often. I then have to bend the truth a little and pretend the Deltics and class 24's remained in service a bit longer. I am a bit of a loco collector, so there will be slots in the timetable for excursions, bringing in class 50's, 26's, 27's, 31's and 37's.
I did consider pretending that Holywell Junction never closed in the 1960's and basing my station there to bring in all the Llandudno services, but then I hit the DMU crisis that you expereinced! I could have brought the class 150 and 156 into service a few years earlier, but that still only got me to five units. At least I can get away with DMU replacement services and use 25's and my fleet of 40's :-)