My main memory starts from when Welwyn Hatfield Line got a number of the ex-Jubilee MCW Metroriders, these later got joined and replaced by Mercedes 709D's and the last WHL route to survive was the WH4 which ran with a new Volvo B6LE.
What actually happened was Sovereign took over Jubilee and then started up a load of local services in Welwyn Garden City - using the 'HR' prefix (for Hertsrider) essentially shadowing the WHL routes, using the ex-Jubilee Metroriders.
This ultimately resulted in WHL being bought out by Sovereign (or its parent company at least), Sovereign withdrawing the 'HR' routes and some fairly significant fleet changes.
As you say, some of the Metroriders were cascaded to WHL. WHL's Optare City Pacers went to County Bus in Harlow and a couple of particularly nasty Ivecos came the other way.
Of WHL's other vehicles - the Bristol coach became Sovereign's training vehicle, the Bedford VAS was sold, the other Bedford coach went to County Bus as their trainer, the Quest went to a charity and the Bedford bus was sold. And the Optare Mercedes they had also disappeared, but not before an additional one had come from Lucketts of Watford.
In their place came 2 Leyland Nationals from Sovereign (223 and 287 ring a bell).
The WHL fleet was 'updated' with Merc 709s and then the name started to disappear.
I think I remember at one time panshanger used to be served by both the WH1 and the WH4. School holidays used to be best as it would be common for a Bristol VR or Leyland Olympian to appear on the route.
Back to the 44, Sovereign certainly ran that again between Seamarks and Grant Palmer, mainly using Lynx G206URO for many years and then B6LE R603WMJ which came from St Albans.
On a side note I was always interested in how Stevenage's vehicles always stayed strongly allocated to certain workings (for example out of their 2 734 workings, Lynx 284 was always on the same one) where as North Mymms and St Albans were more mixed. One week I had the same National 4 days in a row and on 4 different routes (302, 338/9, 366 and 734).
That second "WH1" I think was the replacement for the "G4" doing the Mundells, Herns Way, Moors Walk, Black Fan Road, Mundells loop.
I remember those G2xxURO Lynxes - so much nicer than the SNBs that they replaced.