IIRC the NCN-L buses were converted around the mid 80s to replace the MVK-Rs due to the upcoming deregulation. They NCN-L were replaced by various MCW bodied Leyland Atlanteans around 1992 before they were replaced by ECW Atlanteans 3538/3540, then around 1999 the fleet started to be boosted with vehicles in full DFDS colours.

As mentioned ex Ribble Bristol VRLs were used, owned by the PTE but staffed by Northern drivers.

Examples of vehicles below....
Bristol VRL / ECW - OCK67K - https://www.flickr.com/photos/30794964@N03/8539043756 - Photo by John Evans
Leyland Atlantean / Alexander - MVK505R & MVK500R - https://www.flickr.com/photos/131962719@N04/18314175921 - Photo by Ken Weaver
Leyland Atlantean / ECW - NCN104L (in the yellow and white Northern version of livery) - https://www.flickr.com/photos/busfreakbrian/18178072816 - Photo by Brian H
Leyland Atlantean / ECW - NCN111L (in the red and white Go-Ahead Northern version of the livery) - https://www.flickr.com/photos/henry_patt...9887227551
Leyland Atlantean / MCW - VFT200T (in the red and white Go-Ahead Northern version of the livery with the all white GAN fleetname) - https://www.flickr.com/photos/nedbasher/5694022761 - Photo by Ned Basher
Leyland Atlantean / MCW - VFT185T (in Coastline livery no branding think the ferry companies were in the middle of takeovers) - https://www.flickr.com/photos/solenteer/52094307523 - Photo by Solenteer
Leyland Atlantean / ECW - MBR438T (Coastline livery with Go Coastline Fleetnames and minimal branding, they did originally have Coastline fleetnames) - https://www.flickr.com/photos/thetranspo...2056152913 - Photo by Ernie Robson

Then from as mentioned in 1999/2000 the livery changed to one that was basically a giant advert for DFDS Seaways which to be fair suited the buses. (All photos by me)
Leyland Olympian / ECW - B744GCN - https://www.flickr.com/photos/gjm-photog...3804461055
Volvo Olympian / N-C Palatine 2 - S832OFT - https://www.flickr.com/photos/gjm-photog...1930548033
Dennis Trident / ELC Lolyne - W177SCU - https://www.flickr.com/photos/gjm-photog...0503745722
Dennis Trident / Plaxton President - NK51UCT & NK51UCU - https://www.flickr.com/photos/gjm-photog...1150636104 (This was when some buses went to Metrocentre for the shoppers)

The two tone Blue and White livery was phased out on the low floor fleet (the Volvo Olympian retained it until they became Yellow Bus / Red Spare) for a more advertisment based liveries which I shall use W866PNL as an example showing the different versions it carried, each vehicle has a unique advert to it I think.

The first one was terrible one which basically covered up all the windows on the side as shown here - https://www.flickr.com/photos/gjm-photog...8736088690

The Second Version was a vast improved which allowed those towards the front of the bus to see out of the windows clearly - https://www.flickr.com/photos/gjm-photog...1149060761

Version three was similar to the second version but they added colour to the advert - https://www.flickr.com/photos/gjm-photog...1149286938

Then there is version 4 which meant each bus was white below the downstairs windows and a different colour top - https://www.flickr.com/photos/gjm-photog...5675454157

The Tridents were eventually all replaced with the Omnidekkas leading to Version 5 of the livery using the same base design as version 4 but with different advert graphics as shown by 6145 (524FUP) - https://www.flickr.com/photos/gjm-photog...1174209678

Though I imagine many of the younger enthusiasts on here remember the last 3 versions at least.