A tricky on here.
Top tailgate won’t open. I have replaced the release switch but to no avail. The main tailgate opens, closes and locks OK.
Any ideas before I call in the professionals.

When I had this on my TDV8 the dealership installed a complete new upper section under warranty as "it's too expensive on labour time to dismantle!" The problem was that not only was there dirty water inside between the plastic halves of the upper tailgate from a perished rubber press switch but also what looked like oxidisation on the electrical contacts inside the mechanism. I assume that you have had the condensation/ water ingress issue. The garage implied that they would have to split the two halves of the tailgate to clean it and dry it then replace the wiring and switch that operates the upper section lock so it was cheaper to do as a warranty claim on a complete new upper plastic/ composite section. This is a very common issue on RRS1 and I'm sure one of the experts will come along and tell you shortly. Have you tried using the forum search facillity on the top menu bar? 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW