[Kastellos]

Thanks for the advice. Tried everything but the tailgate remained solidly closed.
In the end we had to destroy the lock to get it open. Came to the conclusion it was a defective lock.
Worked from inside. Unscrewed 2 screws that retained trim grab recess. Pulled carpet trim away from door (it is held at the top by 2 screws that are inaccessible when tailgate closed). Had to work in confined space when trim partly pulled away, both underneath and over the top. Nothing obviously defective. Both the mechanical linkage and the solenoid one were slack and no amount of tugging would make it release the door. Disconnected the electrical plug at the top of the lock. There was no obvious mechanical release mechanism on the outside of the lock. The only option was to destroy the plastic cover over the lock. Removed motors etc. and then got to the bottom metal plate and able with a struggle to rotate the locking pin to release the door.
Got the impression that it was poor engineering. No possibility of releasing mechanically when we did get to the lock and far too much electronics to go wrong.
On the other hand the electrical cable at the hinge has been modified so that it does not tend to fracture on the more recent models.
Lock part number: 31253450 or 40508120 underneath 00308 VOLVO.
Hope others do not have this trouble.